[Runtimes] -nostdlib++ and -nostdinc++ in CMAKE_REQUIRED_FLAGS breaks systems introspection

[Meinersbur]

The flags -nostdlib++ and -nostdinc++ are added to CMAKE_REQUIRED_FLAGS whenever supported by the compiler:

llvm-project/runtimes/CMakeLists.txt

Line 161 in dcd5e47

set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdinc++")

llvm-project/runtimes/CMakeLists.txt

Line 157 in dcd5e47

set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdlib++")

For a normal bootstrapping build the compiler is Clang, so it will always be added. It inherits to all runtimes libraries.

While it may make sense for libc++ to not depend on itself or another C++ standard library, the assumption breaks for any other LLVM runtime library that use C++, which included any GTest-based unittests. Tracking its history, it seems to have first been added in d3ebe71, but just temporarily. The temporary part was dropped at some point. The flags should only be added in those libraries that need it.

Adding those flags globally for all runtimes causes problems in system introspection. e.g.:

• OpenMP: [runtimes] [openmp] -nostdlib++ added to CMAKE_REQUIRED_FLAGS breaks hwloc detection for OMPT #90332
• Flang-RT: Remember LLVM_ENABLE_LIBCXX setting in installed configuration #139712
• Flang-RT: [flang-rt] Fixed HAVE_LDBL_MANT_DIG_113 detection. #131010

[mstorsjo]

For context; I made some effort at fixing this earlier, in #108357 (75d0281), but that had to be reverted.

[Meinersbur]

#108357 is solving a specific problem that C++ flag is also used for check_c_source_compiles which break with gcc. But I don't want -nostdlib++ at all in CMAKE_REQUIRED_FLAGS -- independent of whether it is accepted by the C and/or C++ compiler. The entire underlying assumption that none of the runtime libraries will not use the pre-installed standard library is conceptionally wrong.