Coredump when running global_executor_test

```
[heng@host02-cluster _build]$ ./global_executor_test
[==========] Running 2 tests from 1 test case.
[----------] Global test environment set-up.
[----------] 2 tests from GlobalExecutorTest
[ RUN ] GlobalExecutorTest.GlobalCPUExecutor
[ OK ] GlobalExecutorTest.GlobalCPUExecutor (0 ms)
[ RUN ] GlobalExecutorTest.GlobalIOExecutor
[ OK ] GlobalExecutorTest.GlobalIOExecutor (1 ms)
[----------] 2 tests from GlobalExecutorTest (1 ms total)

[----------] Global test environment tear-down
[==========] 2 tests from 1 test case ran. (2 ms total)
[ PASSED ] 2 tests.
*** Aborted at 1572509134 (Unix time, try 'date -d @1572509134') ***
*** Signal 11 (SIGSEGV) (0xe0) received by PID 30858 (pthread TID 0x7f5b86094700) (linux TID 30860) (code: address not mapped to object), stack trace: ***
@ 000000000086f5af _ZN5folly10symbolizer12_GLOBAL__N_118innerSignalHandlerEiP9siginfo_tPv
/home/heng/workspace/folly/folly/experimental/symbolizer/SignalHandler.cpp:431
@ 000000000086f654 _ZN5folly10symbolizer12_GLOBAL__N_113signalHandlerEiP9siginfo_tPv
/home/heng/workspace/folly/folly/experimental/symbolizer/SignalHandler.cpp:446
@ 00007f5b86e685ef (unknown)
@ 00000000007a02e7 _ZNK5folly14ThreadLocalPtrINS_20SingletonThreadLocalISt10shared_ptrINS_14RequestContextEENS_6detail10DefaultTagENS5_11DefaultMakeIS4_EEvE7WrapperEvvE3getEv
/home/heng/workspace/folly/folly/ThreadLocal.h:159
-> /home/heng/workspace/folly/folly/io/async/Request.cpp
@ 000000000079a797 _ZNK5folly11ThreadLocalINS_20SingletonThreadLocalISt10shared_ptrINS_14RequestContextEENS_6detail10DefaultTagENS5_11DefaultMakeIS4_EEvE7WrapperEvvEdeEv
/home/heng/workspace/folly/folly/ThreadLocal.h:68
-> /home/heng/workspace/folly/folly/io/async/Request.cpp
@ 00000000007980c0 _ZN5folly20SingletonThreadLocalISt10shared_ptrINS_14RequestContextEENS_6detail10DefaultTagENS4_11DefaultMakeIS3_EEvE10getWrapperEv
/home/heng/workspace/folly/folly/SingletonThreadLocal.h:148
-> /home/heng/workspace/folly/folly/io/async/Request.cpp
@ 00000000007980d4 _ZN5folly20SingletonThreadLocalISt10shared_ptrINS_14RequestContextEENS_6detail10DefaultTagENS4_11DefaultMakeIS3_EEvE13LocalLifetimeD1Ev
/home/heng/workspace/folly/folly/SingletonThreadLocal.h:120
-> /home/heng/workspace/folly/folly/io/async/Request.cpp
@ 00007f5b8761eeb5 _ZN12_GLOBAL__N_13runEPv
/home/heng/workspace/gcc-8.3.0/x86_64-pc-linux-gnu/libstdc++-v3/libsupc++/../../.././libstdc++-v3/libsupc++/atexit_thread.cc:75
@ 00007f5b86e60c61 __nptl_deallocate_tsd
@ 00007f5b86e60e72 start_thread
@ 00007f5b86b8988c clone
Segmentation fault (core dumped)
```

version: v2019.10.14.00

Run global_executor_test from the reported build and confirm the shutdown-time SIGSEGV. Start with folly/io/async/Request.cpp, folly/ThreadLocal.h, and folly/SingletonThreadLocal.h, following the stack trace through thread-local cleanup. Done means the test completes without a coredump while both GlobalExecutorTest cases still pass.

Written by the indexing model from the issue text.