[clang] [C++] Expose nullptr_t from stddef.h in C++ mode (PR #154599)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

libc++ failure appears to be unrelated:
```
 FAIL: llvm-libc++-shared.cfg.in :: libcxx/utilities/tuple/no_specializations.verify.cpp (9116 of 10798)
  ******************** TEST 'llvm-libc++-shared.cfg.in :: libcxx/utilities/tuple/no_specializations.verify.cpp' FAILED ********************
  Exit Code: 1
  
  Command Output (stdout):
  --
  # COMPILED WITH
  /home/gha/actions-runner/_work/llvm-project/llvm-project/build/./bin/clang++ /home/gha/actions-runner/_work/llvm-project/llvm-project/libcxx/test/libcxx/utilities/tuple/no_specializations.verify.cpp -pthread --target=x86_64-unknown-linux-gnu -nostdinc++ -I /home/gha/actions-runner/_work/llvm-project/llvm-project/build/runtimes/runtimes-bins/libcxx/test-suite-install/include/x86_64-unknown-linux-gnu/c++/v1 -I /home/gha/actions-runner/_work/llvm-project/llvm-project/build/runtimes/runtimes-bins/libcxx/test-suite-install/include/c++/v1 -I /home/gha/actions-runner/_work/llvm-project/llvm-project/libcxx/test/support -std=c++26 -fmodules -fcxx-modules -fmodules-cache-path=/home/gha/actions-runner/_work/llvm-project/llvm-project/build/runtimes/runtimes-bins/libcxx/test/ModuleCache -Werror -Wall -Wctad-maybe-unsupported -Wextra -Wshadow -Wundef -Wunused-template -Wno-unused-command-line-argument -Wno-attributes -Wno-pessimizing-move -Wno-noexcept-type -Wno-atomic-alignment -Wno-reserved-module-identifier -Wdeprecated-copy -Wdeprecated-copy-dtor -Wshift-negative-value -Wno-user-defined-literals -Wno-tautological-compare -Wsign-compare -Wunused-variable -Wunused-parameter -Wunreachable-code -Wno-unused-local-typedef -Wno-local-type-template-args -Wno-c++11-extensions -Wno-unknown-pragmas -Wno-pass-failed -Wno-mismatched-new-delete -Wno-redundant-move -Wno-self-move -Wno-nullability-completeness -D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER -D_LIBCPP_ENABLE_EXPERIMENTAL -D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_EXTENSIVE -Wuser-defined-warnings  -fsyntax-only -Wno-error -Xclang -verify -Xclang -verify-ignore-unexpected=note -ferror-limit=0
  # executed command: /home/gha/actions-runner/_work/llvm-project/llvm-project/build/./bin/clang++ /home/gha/actions-runner/_work/llvm-project/llvm-project/libcxx/test/libcxx/utilities/tuple/no_specializations.verify.cpp -pthread --target=x86_64-unknown-linux-gnu -nostdinc++ -I /home/gha/actions-runner/_work/llvm-project/llvm-project/build/runtimes/runtimes-bins/libcxx/test-suite-install/include/x86_64-unknown-linux-gnu/c++/v1 -I /home/gha/actions-runner/_work/llvm-project/llvm-project/build/runtimes/runtimes-bins/libcxx/test-suite-install/include/c++/v1 -I /home/gha/actions-runner/_work/llvm-project/llvm-project/libcxx/test/support -std=c++26 -fmodules -fcxx-modules -fmodules-cache-path=/home/gha/actions-runner/_work/llvm-project/llvm-project/build/runtimes/runtimes-bins/libcxx/test/ModuleCache -Werror -Wall -Wctad-maybe-unsupported -Wextra -Wshadow -Wundef -Wunused-template -Wno-unused-command-line-argument -Wno-attributes -Wno-pessimizing-move -Wno-noexcept-type -Wno-atomic-alignment -Wno-reserved-module-identifier -Wdeprecated-copy -Wdeprecated-copy-dtor -Wshift-negative-value -Wno-user-defined-literals -Wno-tautological-compare -Wsign-compare -Wunused-variable -Wunused-parameter -Wunreachable-code -Wno-unused-local-typedef -Wno-local-type-template-args -Wno-c++11-extensions -Wno-unknown-pragmas -Wno-pass-failed -Wno-mismatched-new-delete -Wno-redundant-move -Wno-self-move -Wno-nullability-completeness -D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER -D_LIBCPP_ENABLE_EXPERIMENTAL -D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_EXTENSIVE -Wuser-defined-warnings -fsyntax-only -Wno-error -Xclang -verify -Xclang -verify-ignore-unexpected=note -ferror-limit=0
  # .---command stderr------------
  # | error: 'expected-error' diagnostics expected but not seen: 
  # |   File /home/gha/actions-runner/_work/llvm-project/llvm-project/libcxx/test/libcxx/utilities/tuple/no_specializations.verify.cpp Line 22: cannot be specialized
  # | 1 error generated.
  # `-----------------------------
  # error: command failed with exit status: 1
  
  --
  
  ********************
```


https://github.com/llvm/llvm-project/pull/154599