[runtimes] Check whether -nostdinc++ and -nostdlib++ are supported

[mstorsjo]

Summary

Don't blindly assume they're supported - GCC doesn't support -nostdlib++.

The llvm-project/runtimes directory is supposed to allow building the
runtimes standalone from a newly built Clang, and thus should allow
building with other compilers too.

[mstorsjo]

Reorder the checks to let -nostdlib++ take effect first, as it affects whether other checks succeed or not.

[phosek]

LGTM

[mstorsjo]

Reuploading with a dummy change under libcxx, to trigger rerunning the libcxx premerge tests with this change, will remove that before pushing.

libcxx/CMakeLists.txt

 # See https://libcxx.llvm.org/docs/BuildingLibcxx.html for instructions on how
 # to build libcxx with CMake.
+# Dummy change.
 
 if (NOT IS_DIRECTORY "${CMAKE_CURRENT_LIST_DIR}/../libcxxabi")
   message(FATAL_ERROR "libc++ now requires being built in a monorepo layout with libcxxabi available")
 endif()
 
 #===============================================================================
 # Setup Project
 #===============================================================================

runtimes/CMakeLists.txt

 set(LLVM_PATH ${CMAKE_CURRENT_SOURCE_DIR}/../llvm)
 
 if(APPLE)
   set(LLVM_ENABLE_LIBCXX ON CACHE BOOL "")
 endif()
 
 include(CheckLibraryExists)
 include(CheckCCompilerFlag)
+include(CheckCXXCompilerFlag)
 
-# Disable use of the installed C++ standard library when building runtimes. If
-# MSVC is true, we must be using the clang-cl driver, which doesn't understand
-# these flags.
-if (NOT MSVC)
-  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdinc++ -nostdlib++")
+# Disable use of the installed C++ standard library when building runtimes.
+# Check for -nostdlib++ first; if there's no C++ standard library yet,
+# all check_cxx_compiler_flag commands will fail until we add -nostdlib++
+# (or -nodefaultlibs).
+check_c_compiler_flag(-nostdlib++ LLVM_RUNTIMES_SUPPORT_NOSTDLIBXX_FLAG)
+if (LLVM_RUNTIMES_SUPPORT_NOSTDLIBXX_FLAG)
+  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdlib++")
+endif()
+check_cxx_compiler_flag(-nostdinc++ LLVM_RUNTIMES_SUPPORT_NOSTDINCXX_FLAG)
+if (LLVM_RUNTIMES_SUPPORT_NOSTDINCXX_FLAG)
+  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdinc++")
 endif()
 
 # Avoid checking whether the compiler is working.
 set(LLVM_COMPILER_CHECKED ON)
 
 # Handle common options used by all runtimes.
 include(AddLLVM)
 include(HandleLLVMOptions)