[llvm] [CAS][Unittest] Add a missing include (PR #164593)

[Tomohiro Kashiwada via llvm-commits]

kikairoya wrote:

#114102 broke a buildbot and Cygwin build.

https://lab.llvm.org/buildbot/#/builders/122/builds/2165
```
[515/1163] Building CXX object unittests/CAS/CMakeFiles/CASTests.dir/CASTestConfig.cpp.o
FAILED: unittests/CAS/CMakeFiles/CASTests.dir/CASTestConfig.cpp.o 
/usr/local/bin/c++ -DGTEST_HAS_PTHREAD=0 -DGTEST_HAS_RTTI=0 -DLLVM_BUILD_STATIC -D_DEBUG -D_FILE_OFFSET_BITS=64 -D_GLIBCXX_ASSERTIONS -D_GLIBCXX_USE_CXX11_ABI=1 -D_GNU_SOURCE -D_LARGEFILE_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/home/tcwg-buildbot/worker/clang-armv8-lld-2stage/stage1/unittests/CAS -I/home/tcwg-buildbot/worker/clang-armv8-lld-2stage/llvm/llvm/unittests/CAS -I/home/tcwg-buildbot/worker/clang-armv8-lld-2stage/stage1/include -I/home/tcwg-buildbot/worker/clang-armv8-lld-2stage/llvm/llvm/include -I/home/tcwg-buildbot/worker/clang-armv8-lld-2stage/llvm/third-party/unittest/googletest/include -I/home/tcwg-buildbot/worker/clang-armv8-lld-2stage/llvm/third-party/unittest/googlemock/include -mcpu=neoverse-n1 -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -pedantic -Wno-long-long -Wc++98-compat-extra-semi -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wstring-conversion -Wno-pass-failed -Wmisleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -ffunction-sections -fdata-sections -O3 -DNDEBUG -std=c++17  -Wno-variadic-macros -Wno-gnu-zero-variadic-macro-arguments -fno-exceptions -funwind-tables -fno-rtti -UNDEBUG -Wno-suggest-override -MD -MT unittests/CAS/CMakeFiles/CASTests.dir/CASTestConfig.cpp.o -MF unittests/CAS/CMakeFiles/CASTests.dir/CASTestConfig.cpp.o.d -o unittests/CAS/CMakeFiles/CASTests.dir/CASTestConfig.cpp.o -c /home/tcwg-buildbot/worker/clang-armv8-lld-2stage/llvm/llvm/unittests/CAS/CASTestConfig.cpp
../llvm/llvm/unittests/CAS/CASTestConfig.cpp:29:10: error: no type named 'once_flag' in namespace 'std'; did you mean '__once_flag'?
   29 |   static std::once_flag Flag;
      |          ^~~~~~~~~~~~~~
      |          __once_flag
/usr/include/arm-linux-gnueabihf/bits/thread-shared-types.h:111:3: note: '__once_flag' declared here
  111 | } __once_flag;
      |   ^
../llvm/llvm/unittests/CAS/CASTestConfig.cpp:30:8: error: no member named 'call_once' in namespace 'std'; did you mean 'calloc'?
   30 |   std::call_once(
      |        ^~~~~~~~~
      |        calloc
/usr/lib/gcc/arm-linux-gnueabihf/11/../../../../include/c++/11/cstdlib:144:11: note: 'calloc' declared here
  144 |   using ::calloc;
      |           ^
2 errors generated.
```

@cachemeifyoucan @adrian-prantl @ilovepi 
Could you please take a look for this?

https://github.com/llvm/llvm-project/pull/164593