[CMake] [Runtimes] Set *_STANDALONE_BUILD

[beanz]

Summary

The runtimes use *_STANDALONE_BUILD=OFF to signify that clang is an in-tree target. This is not the case with the runtime builds, so we really need this set to ON.

In order to resolve the issues phosek was having with checks, we should use checks that don't link. We can use compiler-rt's try_compile_only as a basis for that.

This patch is *required* to be able to run the runtime libraries check-* targets.

[phosek]

This is fine with me but I'd first like to test this against our build to make sure it doesn't break anything.

[phosek]

LGTM seems to be working fine in our build.

llvm/runtimes/CMakeLists.txt

if(${CMAKE_SOURCE_DIR} STREQUAL ${CMAKE_CURRENT_SOURCE_DIR})
 
   foreach(entry ${runtimes})
     get_filename_component(projName ${entry} NAME)
 
     # TODO: Clean this up as part of an interface standardization
     string(REPLACE "-" "_" canon_name ${projName})
     string(TOUPPER ${canon_name} canon_name)
 
+    # The subdirectories need to treat this as standalone builds. D57992 tried
+    # to get rid of this, but the runtimes treat *_STANDALONE_BUILD=OFF as if
+    # llvm & clang are configured in the same CMake, and setup dependencies
+    # against their targets.
+    set(${canon_name}_STANDALONE_BUILD ON)
+
     if(LLVM_RUNTIMES_LIBDIR_SUBDIR)
       set(${canon_name}_LIBDIR_SUBDIR "${LLVM_RUNTIMES_LIBDIR_SUBDIR}" CACHE STRING "" FORCE)
     endif()
 
     # Setting a variable to let sub-projects detect which other projects
     # will be included under here.
     set(HAVE_${canon_name} ON)
   endforeach()