Allow function to not be set in BaseException... introduces regression

### HHVM Version

swalk@dellx3:~/play.032219a/hhvm$ hphp/hhvm/hhvm --version
HipHop VM 4.1.0-dev (rel)
Compiler: heads/master-0-gb66b2e56797989d73f1e8c0346ae28b37f20dcdf
Repo schema: bb0dccc4d3fb2303ea3da93ccf675403fa84782c

### Operating System and Version

Ubuntu 16.04, Intel Haswell box

swalk@dellx3:~/play.032219a/hhvm$ uname -a
Linux dellx3 4.4.0-57-generic #78-Ubuntu SMP Fri Dec 9 23:50:32 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux

### Standalone code, or other way to reproduce the problem

hphp/test/run -v -m interp
hphp/test/run -v -m interp -r
hphp/test/run -v -m jit -a '-vEval.JitPGO=0'
hphp/test/run -v -m jit -r -a '-vEval.JitPGO=0'
hphp/test/run -v -m jit -a '-vEval.JitPGO=1'
hphp/test/run -v -m jit -r -a '-vEval.JitPGO=1'

### Actual result

Post-processed report below. Columns line up with option sets above.

hphp/test/slow/exceptions/gen_user_func_exception_trace.php | F | F | F | F | F | F |

### Expected result

Pass

This regression showed up in my testing of 03/22/2019
Suspect change was introduced by this commit
https://github.com/facebook/hhvm/commit/d43fb1461b858cdf9caf6578062df40af960c126

Test references internal Facebook function?

swalk@dellx3:~/play.032219a/hhvm$ ./hphp/hhvm/hhvm -c /home/swalk/play.032219a/hhvm/hphp/test/slow/config.ini -vEval.EnableArgsInBacktraces=true -vEval.EnableIntrinsicsExtension=true -vEval.HHIRInliningIgnoreHints=false -vRepo.Local.Mode=-- -vRepo.Central.Path=/home/swalk/play.032219a/hhvm/hphp/hhvm/verify.hhbc -vEval.Jit=0 -vEval.ProfileHWEnable=false -vEval.HackCompilerExtractPath='/home/swalk/play.032219a/hhvm/hphp/hhvm/hackc_%{schema}' -vEval.EmbeddedDataExtractPath='/home/swalk/play.032219a/hhvm/hphp/hhvm/hhvm_%{type}_%{buildid}' -vResourceLimit.CoreFileSize=0 --file 'hphp/test/slow/exceptions/gen_user_func_exception_trace.php'

Fatal error: Call to undefined function fb_gen_user_func_array() in /home/swalk/play.032219a/hhvm/hphp/test/slow/exceptions/gen_user_func_exception_trace.php on line 15

Start with hphp/test/slow/exceptions/gen_user_func_exception_trace.php and run the six hphp/test/run commands listed in the issue across interpreter and JIT modes. Compare the test behavior with commit d43fb1461b858cdf9caf6578062df40af960c126 and determine why the regression occurs; done means the test passes in all listed configurations.

Written by the indexing model from the issue text.