[clang] flaky unit test TimeProfilerTest.ConstantEvaluationC99

Occasionally (1 in 10 chance or so) this test fails for no reason, on a MacOS host:

```
******************** TEST 'Clang-Unit :: ./AllClangUnitTests/29/51' FAILED ********************
Script(shard):
--
GTEST_OUTPUT=json:build/rwd/tools/clang/unittests/./AllClangUnitTests-Clang-Unit-52457-29-51.json GTEST_SHUFFLE=0 GTEST_TOTAL_SHARDS=51 GTEST_SHARD_INDEX=29 build/rwd/tools/clang/unittests/./AllClangUnitTests
--

Script:
--
build/rwd/tools/clang/unittests/./AllClangUnitTests --gtest_filter=TimeProfilerTest.ConstantEvaluationC99
--
clang/unittests/Support/TimeProfilerTest.cpp:366: Failure
Expected equality of these values:
R"(
ExecuteCompiler
| Frontend (test.c)
| | ParseDeclarationOrFunctionDefinition (test.c:2:1)
| | | isIntegerConstantExpr ()
| | | EvaluateKnownConstIntCheckOverflow ()
| PerformPendingInstantiations
)"
Which is: "\nExecuteCompiler\n| Frontend (test.c)\n| | ParseDeclarationOrFunctionDefinition (test.c:2:1)\n| | | isIntegerConstantExpr ()\n| | | EvaluateKnownConstIntCheckOverflow ()\n| PerformPendingInstantiations\n"
buildTraceGraph(Json)
Which is: "\nExecuteCompiler\n| Frontend (test.c)\n| | ParseDeclarationOrFunctionDefinition (test.c:2:1)\n| | | isIntegerConstantExpr ()\n| | | EvaluateKnownConstIntCheckOverflow ()\n| | | PerformPendingInstantiations\n"
With diff:
@@ -5,3 +5,3 @@
| | | isIntegerConstantExpr ()
| | | EvaluateKnownConstIntCheckOverflow ()
-| PerformPendingInstantiations\n
+| | | PerformPendingInstantiations\n

clang/unittests/Support/TimeProfilerTest.cpp:366
Expected equality of these values:
R"(
ExecuteCompiler
| Frontend (test.c)
| | ParseDeclarationOrFunctionDefinition (test.c:2:1)
| | | isIntegerConstantExpr ()
| | | EvaluateKnownConstIntCheckOverflow ()
| PerformPendingInstantiations
)"
Which is: "\nExecuteCompiler\n| Frontend (test.c)\n| | ParseDeclarationOrFunctionDefinition (test.c:2:1)\n| | | isIntegerConstantExpr ()\n| | | EvaluateKnownConstIntCheckOverflow ()\n| PerformPendingInstantiations\n"
buildTraceGraph(Json)
Which is: "\nExecuteCompiler\n| Frontend (test.c)\n| | ParseDeclarationOrFunctionDefinition (test.c:2:1)\n| | | isIntegerConstantExpr ()\n| | | EvaluateKnownConstIntCheckOverflow ()\n| | | PerformPendingInstantiations\n"
With diff:
@@ -5,3 +5,3 @@
| | | isIntegerConstantExpr ()
| | | EvaluateKnownConstIntCheckOverflow ()
-| PerformPendingInstantiations\n
+| | | PerformPendingInstantiations\n
```

Start with clang/unittests/Support/TimeProfilerTest.cpp at line 366 and run the TimeProfilerTest.ConstantEvaluationC99 filter repeatedly on a macOS host. Compare the expected trace with buildTraceGraph(Json), then investigate why PerformPendingInstantiations sometimes has a different nesting level. Done means the test no longer flakes and its trace remains consistent.

Written by the indexing model from the issue text.