[MSVC] Don't add -nostdinc++ -isystem to runtimes builds

[rnk]

Summary

If the host compiler is MSVC or clang-cl, then the compiler used to
buidl the runtimes will be clang-cl, and it doesn't support either of
those flags.

Worse, because -isystem is a space separated flag, it causes all cmake
try_compile tests to fail, so none of the -Wno-* flags make it to the
compiler in libcxx. I noticed that we weren't passing
-Wno-user-defined-literals to clang-cl and were getting warnings in the
build, and this fixes that for me.

[mstorsjo]

runtimes/CMakeLists.txt

81

Doesn't the runtimes build system support cross compiling runtimes? (I don't use it myself as I need to build a number of other components between building the compiler and building the llvm provided runtimes, but afaik it's possible.) In that case, the logic for deducing things from the host compiler wouldn't apply...

[rnk]

runtimes/CMakeLists.txt

81

You're right, this comment is incorrect, I think this CMakeLists.txt file is configured using the just-built clang. I had misunderstood how the runtimes build is put together. If we get here and MSVC is true, we must be using clang-cl, but the actual code change still makes sense.

[rnk]

• fix misleading comment

[mstorsjo]

LGTM

runtimes/CMakeLists.txt

 
 if(APPLE)
   set(LLVM_ENABLE_LIBCXX ON CACHE BOOL "")
 endif()
 
 include(CheckLibraryExists)
 include(CheckCCompilerFlag)
 
-# We don't have libc++ (yet)...
+# 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++")
 
-# ...but we need access to libc++ headers for CMake checks to succeed.
   if (LLVM_EXTERNAL_LIBCXX_SOURCE_DIR AND "libcxx" IN_LIST LLVM_ENABLE_RUNTIMES)
     set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -isystem ${LLVM_EXTERNAL_LIBCXX_SOURCE_DIR}/include")
   endif()
+endif()
 
 # Avoid checking whether the compiler is working.
 set(LLVM_COMPILER_CHECKED ON)
 
 # Handle common options used by all runtimes.
 include(AddLLVM)
 include(HandleLLVMOptions)
 include(FindPythonInterp)