Shaded Checker Framework dataflow library can crash for JDK12-JDK15 (which are among the versions it doesn't support)

Since no one else seems to have hit this, I don't see much need to address it. But if it's worth a look someday, we could try skipping dataflow for JDK 12, 13, 14, and 15. Of course, that works fine for a check like `AbstractReferenceEquality` that uses dataflow only to generate simpler fixes—but it wouldn't be a good option if a check _relies_ on dataflow.

Mostly I'm filing this bug because I feel like this stack trace should appear _somewhere_ on the Internet for anyone who might see it again. I also put some notes about Error Prone at the end of https://github.com/google/guava/pull/6243.

```
An exception has occurred in the compiler (15.0.2). Please file a bug against the Java compiler via the Java bug reporting page (http://bugreport.java.com) after checking the Bug Database (http://bugs.java.com) for duplicates. Include your program, the following diagnostic, and the parameters passed to the Java compiler in your report. Thank you.
com.google.common.util.concurrent.ExecutionError: java.lang.ExceptionInInitializerError
at com.google.common.cache.LocalCache$Segment.get(LocalCache.java:2053)
at com.google.common.cache.LocalCache.get(LocalCache.java:3966)
at com.google.common.cache.LocalCache.getOrLoad(LocalCache.java:3989)
at com.google.common.cache.LocalCache$LocalLoadingCache.get(LocalCache.java:4950)
at com.google.common.cache.LocalCache$LocalLoadingCache.getUnchecked(LocalCache.java:4956)
at com.google.errorprone.dataflow.DataFlow.methodDataflow(DataFlow.java:176)
at com.google.errorprone.dataflow.DataFlow.expressionDataflow(DataFlow.java:236)
at com.google.errorprone.dataflow.nullnesspropagation.NullnessAnalysis.getNullness(NullnessAnalysis.java:59)
at com.google.errorprone.bugpatterns.AbstractReferenceEquality.getNullness(AbstractReferenceEquality.java:224)
at com.google.errorprone.bugpatterns.AbstractReferenceEquality.addFixes(AbstractReferenceEquality.java:128)
at com.google.errorprone.bugpatterns.AbstractReferenceEquality.matchBinary(AbstractReferenceEquality.java:83)
at com.google.errorprone.scanner.ErrorProneScanner.processMatchers(ErrorProneScanner.java:449)
at com.google.errorprone.scanner.ErrorProneScanner.visitBinary(ErrorProneScanner.java:512)
at com.google.errorprone.scanner.ErrorProneScanner.visitBinary(ErrorProneScanner.java:150)
at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCBinary.accept(JCTree.java:2114)
at jdk.compiler/com.sun.source.util.TreePathScanner.scan(TreePathScanner.java:82)
at com.google.errorprone.scanner.Scanner.scan(Scanner.java:74)
at com.google.errorprone.scanner.Scanner.scan(Scanner.java:48)
at jdk.compiler/com.sun.source.util.TreeScanner.scanAndReduce(TreeScanner.java:90)
at jdk.compiler/com.sun.source.util.TreeScanner.visitBinary(TreeScanner.java:644)
at com.google.errorprone.scanner.ErrorProneScanner.visitBinary(ErrorProneScanner.java:513)
at com.google.errorprone.scanner.ErrorProneScanner.visitBinary(ErrorProneScanner.java:150)
at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCBinary.accept(JCTree.java:2114)
at jdk.compiler/com.sun.source.util.TreePathScanner.scan(TreePathScanner.java:82)
at com.google.errorprone.scanner.Scanner.scan(Scanner.java:74)
at com.google.errorprone.scanner.Scanner.scan(Scanner.java:48)
at jdk.compiler/com.sun.source.util.TreeScanner.visitBinary(TreeScanner.java:643)
at com.google.errorprone.scanner.ErrorProneScanner.visitBinary(ErrorProneScanner.java:513)
at com.google.errorprone.scanner.ErrorProneScanner.visitBinary(ErrorProneScanner.java:150)
at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCBinary.accept(JCTree.java:2114)
at jdk.compiler/com.sun.source.util.TreePathScanner.scan(TreePathScanner.java:82)
at com.google.errorprone.scanner.Scanner.scan(Scanner.java:74)
at com.google.errorprone.scanner.Scanner.scan(Scanner.java:48)
at jdk.compiler/com.sun.source.util.TreeScanner.visitParenthesized(TreeScanner.java:591)
at com.google.errorprone.scanner.ErrorProneScanner.visitParenthesized(ErrorProneScanner.java:799)
at com.google.errorprone.scanner.ErrorProneScanner.visitParenthesized(ErrorProneScanner.java:150)
at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCParens.accept(JCTree.java:1970)
at jdk.compiler/com.sun.source.util.TreePathScanner.scan(TreePathScanner.java:82)
at com.google.errorprone.scanner.Scanner.scan(Scanner.java:74)
at com.google.errorprone.scanner.Scanner.scan(Scanner.java:48)
at jdk.compiler/com.sun.source.util.TreeScanner.visitIf(TreeScanner.java:437)
at com.google.errorprone.scanner.ErrorProneScanner.visitIf(ErrorProneScanner.java:654)
at com.google.errorprone.scanner.ErrorProneScanner.visitIf(ErrorProneScanner.java:150)
at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCIf.accept(JCTree.java:1513)
at jdk.compiler/com.sun.source.util.TreePathScanner.scan(TreePathScanner.java:82)
at com.google.errorprone.scanner.Scanner.scan(Scanner.java:74)
at com.google.errorprone.scanner.Scanner.scan(Scanner.java:48)
at jdk.compiler/com.sun.source.util.TreeScanner.scanAndReduce(TreeScanner.java:90)
at jdk.compiler/com.sun.source.util.TreeScanner.scan(TreeScanner.java:105)
at jdk.compiler/com.sun.source.util.TreeScanner.visitBlock(TreeScanner.java:250)
at com.google.errorprone.scanner.ErrorProneScanner.visitBlock(ErrorProneScanner.java:520)
at com.google.errorprone.scanner.ErrorProneScanner.visitBlock(ErrorProneScanner.java:150)
at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCBlock.accept(JCTree.java:1059)
at jdk.compiler/com.sun.source.util.TreePathScanner.scan(TreePathScanner.java:82)
at com.google.errorprone.scanner.Scanner.scan(Scanner.java:74)
at com.google.errorprone.scanner.Scanner.scan(Scanner.java:48)
at jdk.compiler/com.sun.source.util.TreeScanner.scanAndReduce(TreeScanner.java:90)
at jdk.compiler/com.sun.source.util.TreeScanner.visitMethod(TreeScanner.java:208)
at com.google.errorprone.scanner.ErrorProneScanner.visitMethod(ErrorProneScanner.java:740)
at com.google.errorprone.scanner.ErrorProneScanner.visitMethod(ErrorProneScanner.java:150)
at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCMethodDecl.accept(JCTree.java:925)
at jdk.compiler/com.sun.source.util.TreePathScanner.scan(TreePathScanner.java:82)
at com.google.errorprone.scanner.Scanner.scan(Scanner.java:74)
at com.google.errorprone.scanner.Scanner.scan(Scanner.java:48)
at jdk.compiler/com.sun.source.util.TreeScanner.scanAndReduce(TreeScanner.java:90)
at jdk.compiler/com.sun.source.util.TreeScanner.scan(TreeScanner.java:105)
at jdk.compiler/com.sun.source.util.TreeScanner.scanAndReduce(TreeScanner.java:113)
at jdk.compiler/com.sun.source.util.TreeScanner.visitClass(TreeScanner.java:189)
at com.google.errorprone.scanner.ErrorProneScanner.visitClass(ErrorProneScanner.java:548)
at com.google.errorprone.scanner.ErrorProneScanner.visitClass(ErrorProneScanner.java:150)
at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCClassDecl.accept(JCTree.java:832)
at jdk.compiler/com.sun.source.util.TreePathScanner.scan(TreePathScanner.java:82)
at com.google.errorprone.scanner.Scanner.scan(Scanner.java:74)
at com.google.errorprone.scanner.Scanner.scan(Scanner.java:48)
at jdk.compiler/com.sun.source.util.TreeScanner.scan(TreeScanner.java:105)
at jdk.compiler/com.sun.source.util.TreeScanner.scanAndReduce(TreeScanner.java:113)
at jdk.compiler/com.sun.source.util.TreeScanner.visitCompilationUnit(TreeScanner.java:144)
at com.google.errorprone.scanner.ErrorProneScanner.visitCompilationUnit(ErrorProneScanner.java:560)
at com.google.errorprone.scanner.ErrorProneScanner.visitCompilationUnit(ErrorProneScanner.java:150)
at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCCompilationUnit.accept(JCTree.java:603)
at jdk.compiler/com.sun.source.util.TreePathScanner.scan(TreePathScanner.java:56)
at com.google.errorprone.scanner.Scanner.scan(Scanner.java:58)
at com.google.errorprone.scanner.ErrorProneScannerTransformer.apply(ErrorProneScannerTransformer.java:43)
at com.google.errorprone.ErrorProneAnalyzer.finished(ErrorProneAnalyzer.java:152)
at jdk.compiler/com.sun.tools.javac.api.MultiTaskListener.finished(MultiTaskListener.java:132)
at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.flow(JavaCompiler.java:1421)
at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.flow(JavaCompiler.java:1368)
at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:960)
at jdk.compiler/com.sun.tools.javac.main.Main.compile(Main.java:317)
at jdk.compiler/com.sun.tools.javac.main.Main.compile(Main.java:176)
at jdk.compiler/com.sun.tools.javac.Main.compile(Main.java:59)
at jdk.compiler/com.sun.tools.javac.Main.main(Main.java:45)
Caused by: java.lang.ExceptionInInitializerError
at org.checkerframework.errorprone.dataflow.cfg.builder.CFGTranslationPhaseOne.visitMethodInvocation(CFGTranslationPhaseOne.java:1402)
at org.checkerframework.errorprone.dataflow.cfg.builder.CFGTranslationPhaseOne.visitMethodInvocation(CFGTranslationPhaseOne.java:197)
at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCMethodInvocation.accept(JCTree.java:1761)
at org.checkerframework.errorprone.dataflow.cfg.builder.CFGTranslationPhaseOne.scan(CFGTranslationPhaseOne.java:545)
at org.checkerframework.errorprone.dataflow.cfg.builder.CFGTranslationPhaseOne.visitExpressionStatement(CFGTranslationPhaseOne.java:2679)
at org.checkerframework.errorprone.dataflow.cfg.builder.CFGTranslationPhaseOne.visitExpressionStatement(CFGTranslationPhaseOne.java:197)
at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCExpressionStatement.accept(JCTree.java:1540)
at org.checkerframework.errorprone.dataflow.cfg.builder.CFGTranslationPhaseOne.scan(CFGTranslationPhaseOne.java:545)
at org.checkerframework.errorprone.dataflow.cfg.builder.CFGTranslationPhaseOne.visitBlock(CFGTranslationPhaseOne.java:2201)
at org.checkerframework.errorprone.dataflow.cfg.builder.CFGTranslationPhaseOne.visitBlock(CFGTranslationPhaseOne.java:197)
at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCBlock.accept(JCTree.java:1059)
at org.checkerframework.errorprone.dataflow.cfg.builder.CFGTranslationPhaseOne.scan(CFGTranslationPhaseOne.java:545)
at org.checkerframework.errorprone.dataflow.cfg.builder.CFGTranslationPhaseOne.process(CFGTranslationPhaseOne.java:442)
at org.checkerframework.errorprone.dataflow.cfg.builder.CFGTranslationPhaseOne.process(CFGTranslationPhaseOne.java:493)
at org.checkerframework.errorprone.dataflow.cfg.builder.CFGBuilder.build(CFGBuilder.java:70)
at com.google.errorprone.dataflow.DataFlow$2.load(DataFlow.java:127)
at com.google.errorprone.dataflow.DataFlow$2.load(DataFlow.java:95)
at com.google.common.cache.LocalCache$LoadingValueReference.loadFuture(LocalCache.java:3533)
at com.google.common.cache.LocalCache$Segment.loadSync(LocalCache.java:2282)
at com.google.common.cache.LocalCache$Segment.lockedGetOrLoad(LocalCache.java:2159)
at com.google.common.cache.LocalCache$Segment.get(LocalCache.java:2049)
... 91 more
Caused by: org.checkerframework.errorprone.javacutil.BugInCF: JDK 12+ reflection problem
at org.checkerframework.errorprone.javacutil.TreeUtils.(TreeUtils.java:144)
... 112 more
Caused by: java.lang.NoSuchMethodException: com.sun.source.tree.BindingPatternTree.getVariable()
at java.base/java.lang.Class.getMethod(Class.java:2201)
at org.checkerframework.errorprone.javacutil.TreeUtils.(TreeUtils.java:136)
... 112 more
```