[CMake] Don't use libc++ by default on Windows yet

[phosek]

Summary

libc++ has issues when used with -fno-exceptions and vcruntime,
don't use it on Windows by default until we address those issues.

clang/cmake/caches/Fuchsia-stage2.cmake

 set(LLVM_INCLUDE_GO_TESTS OFF CACHE BOOL "")
 set(LLVM_STATIC_LINK_CXX_STDLIB ON CACHE BOOL "")
 set(LLVM_USE_RELATIVE_PATHS_IN_FILES ON CACHE BOOL "")
 
 if(WIN32)
   set(LLVM_USE_CRT_RELEASE "MT" CACHE STRING "")
 endif()
 
+if(NOT WIN32)
   set(CLANG_DEFAULT_CXX_STDLIB libc++ CACHE STRING "")
+endif()
 if(NOT APPLE)
   # TODO: Remove this once we switch to ld64.lld.
   set(CLANG_DEFAULT_LINKER lld CACHE STRING "")
   set(CLANG_DEFAULT_OBJCOPY llvm-objcopy CACHE STRING "")
 endif()
 set(CLANG_DEFAULT_RTLIB compiler-rt CACHE STRING "")
 set(CLANG_ENABLE_ARCMT OFF CACHE BOOL "")
 set(CLANG_ENABLE_STATIC_ANALYZER ON CACHE BOOL "")
if(WIN32)
   list(APPEND RUNTIME_TARGETS "${target}")
   set(RUNTIMES_${target}_CMAKE_SYSTEM_NAME Windows CACHE STRING "")
   set(RUNTIMES_${target}_CMAKE_BUILD_TYPE RelWithDebInfo CACHE STRING "")
   set(RUNTIMES_${target}_LIBCXX_ABI_VERSION 2 CACHE STRING "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_FILESYSTEM OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_ABI_LINKER_SCRIPT OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_SHARED OFF CACHE BOOL "")
-  set(RUNTIMES_${target}_LIBCXX_NO_VCRUNTIME ON CACHE BOOL "")
   set(RUNTIMES_${target}_LLVM_ENABLE_RUNTIMES "compiler-rt;libcxx" CACHE STRING "")
 endif()
 
 foreach(target aarch64-unknown-linux-gnu;armv7-unknown-linux-gnueabihf;i386-unknown-linux-gnu;x86_64-unknown-linux-gnu)
   if(LINUX_${target}_SYSROOT)
     # Set the per-target builtins options.
     list(APPEND BUILTIN_TARGETS "${target}")
     set(BUILTINS_${target}_CMAKE_SYSTEM_NAME Linux CACHE STRING "")

clang/cmake/caches/Fuchsia.cmake

 set(LLVM_INCLUDE_DOCS OFF CACHE BOOL "")
 set(LLVM_INCLUDE_EXAMPLES OFF CACHE BOOL "")
 set(LLVM_INCLUDE_GO_TESTS OFF CACHE BOOL "")
 
 if(WIN32)
   set(LLVM_USE_CRT_RELEASE "MT" CACHE STRING "")
 endif()
 
+if(NOT WIN32)
   set(CLANG_DEFAULT_CXX_STDLIB libc++ CACHE STRING "")
+endif()
 if(NOT APPLE)
   # TODO: Remove this once we switch to ld64.lld.
   set(CLANG_DEFAULT_LINKER lld CACHE STRING "")
   set(CLANG_DEFAULT_OBJCOPY llvm-objcopy CACHE STRING "")
 endif()
 set(CLANG_DEFAULT_RTLIB compiler-rt CACHE STRING "")
 set(CLANG_ENABLE_ARCMT OFF CACHE BOOL "")
 set(CLANG_ENABLE_STATIC_ANALYZER ON CACHE BOOL "")

 set(LIBCXX_ABI_VERSION 2 CACHE STRING "")
 set(LIBCXX_ENABLE_SHARED OFF CACHE BOOL "")
 if(WIN32)
   set(LIBCXX_HAS_WIN32_THREAD_API ON CACHE BOOL "")
   set(LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY OFF CACHE BOOL "")
   set(LIBCXX_ENABLE_FILESYSTEM OFF CACHE BOOL "")
   set(LIBCXX_ENABLE_ABI_LINKER_SCRIPT OFF CACHE BOOL "")
   set(LIBCXX_ENABLE_STATIC_ABI_LIBRARY OFF CACHE BOOL "")
-  set(LIBCXX_NO_VCRUNTIME ON CACHE BOOL "")
   set(BUILTINS_CMAKE_ARGS -DCMAKE_SYSTEM_NAME=Windows CACHE STRING "")
   set(RUNTIMES_CMAKE_ARGS -DCMAKE_SYSTEM_NAME=Windows CACHE STRING "")
   set(LLVM_ENABLE_RUNTIMES "compiler-rt;libcxx" CACHE STRING "")
 else()
   set(LIBCXX_ENABLE_STATIC_ABI_LIBRARY ON CACHE BOOL "")
   set(LLVM_ENABLE_RUNTIMES "compiler-rt;libcxx;libcxxabi;libunwind" CACHE STRING "")
 endif()