[CMake] Support builtins as Clang default rtlib in compiler-rt

[phosek]

Summary

Use compiler-rt builtins when selected as default Clang rtlib and avoid
explicitly passing -rtlib= flag to avoid the "argument unused during
compilation" warning.

This is a partial alternative to D47094 that does not rely on compiler
runtime checks.

compiler-rt/CMakeLists.txt

if (SANITIZER_CXX_ABI_INTREE)
     endif()
   else()
     list(APPEND SANITIZER_CXX_ABI_LIBRARY "c++")
   endif()
 elseif (SANITIZER_CXX_ABI_LIBNAME STREQUAL "libstdc++")
   append_list_if(COMPILER_RT_HAS_LIBSTDCXX stdc++ SANITIZER_CXX_ABI_LIBRARY)
 endif()
 
-option(SANITIZER_USE_COMPILER_RT "Use compiler-rt builtins instead of libgcc" ON)
+set(COMPILER_RT_RUNTIME "default" CACHE STRING
+    "Compiler runtime to use.")
+
+if (COMPILER_RT_RUNTIME STREQUAL "default")
+  if (CLANG_DEFAULT_RTLIB STREQUAL "compiler-rt")
+    set(COMPILER_RT_RUNTIME_LIBRARY "builtins")
+  elseif (FUCHSIA)
+    set(COMPILER_RT_RUNTIME_LIBRARY "builtins")
+  endif()
+else()
+  set(COMPILER_RT_RUNTIME_LIBRARY "${COMPILER_RT_RUNTIME}")
+endif()
 
-include(HandleCompilerRT)
 include(config-ix)
 
 #================================
 # Setup Compiler Flags
 #================================
 
 if(MSVC)
   # Override any existing /W flags with /W4. This is what LLVM does.  Failing to

 append_list_if(COMPILER_RT_HAS_WD4291_FLAG /wd4291 SANITIZER_COMMON_CFLAGS)
 append_list_if(COMPILER_RT_HAS_WD4391_FLAG /wd4391 SANITIZER_COMMON_CFLAGS)
 append_list_if(COMPILER_RT_HAS_WD4722_FLAG /wd4722 SANITIZER_COMMON_CFLAGS)
 append_list_if(COMPILER_RT_HAS_WD4800_FLAG /wd4800 SANITIZER_COMMON_CFLAGS)
 
 # Set common link flags.
 append_list_if(COMPILER_RT_HAS_NODEFAULTLIBS_FLAG -nodefaultlibs SANITIZER_COMMON_LINK_FLAGS)
 
-if (SANITIZER_USE_COMPILER_RT)
-  list(APPEND SANITIZER_COMMON_LINK_FLAGS -rtlib=compiler-rt)
-  find_compiler_rt_library(builtins COMPILER_RT_BUILTINS_LIBRARY)
+if (COMPILER_RT_RUNTIME_LIBRARY STREQUAL "builtins")
   list(APPEND SANITIZER_COMMON_LINK_LIBS ${COMPILER_RT_BUILTINS_LIBRARY})
 else()
   if (ANDROID)
     append_list_if(COMPILER_RT_HAS_GCC_LIB gcc SANITIZER_COMMON_LINK_LIBS)
   else()
     append_list_if(COMPILER_RT_HAS_GCC_S_LIB gcc_s SANITIZER_COMMON_LINK_LIBS)
   endif()
 endif()

compiler-rt/cmake/config-ix.cmake

 include(CMakePushCheckState)
 include(CheckCCompilerFlag)
 include(CheckCXXCompilerFlag)
 include(CheckLibraryExists)
 include(CheckSymbolExists)
 include(TestBigEndian)
 
 function(check_linker_flag flag out_var)
   cmake_push_check_state()
   set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} ${flag}")
   check_cxx_compiler_flag("" ${out_var})
   cmake_pop_check_state()
 endfunction()
 
 check_library_exists(c fopen "" COMPILER_RT_HAS_LIBC)
-if (NOT SANITIZER_USE_COMPILER_RT)
+if (COMPILER_RT_RUNTIME_LIBRARY STREQUAL "builtins")
+  include(HandleCompilerRT)
+  find_compiler_rt_library(builtins COMPILER_RT_BUILTINS_LIBRARY)
+else()
   if (ANDROID)
     check_library_exists(gcc __gcc_personality_v0 "" COMPILER_RT_HAS_GCC_LIB)
   else()
     check_library_exists(gcc_s __gcc_personality_v0 "" COMPILER_RT_HAS_GCC_S_LIB)
   endif()
 endif()
 
 check_c_compiler_flag(-nodefaultlibs COMPILER_RT_HAS_NODEFAULTLIBS_FLAG)
 if (COMPILER_RT_HAS_NODEFAULTLIBS_FLAG)
   set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nodefaultlibs")
   if (COMPILER_RT_HAS_LIBC)
     list(APPEND CMAKE_REQUIRED_LIBRARIES c)
   endif ()
-  if (SANITIZER_USE_COMPILER_RT)
-    list(APPEND CMAKE_REQUIRED_FLAGS -rtlib=compiler-rt)
-    find_compiler_rt_library(builtins COMPILER_RT_BUILTINS_LIBRARY)
+  if (COMPILER_RT_RUNTIME_LIBRARY STREQUAL "builtins")
     list(APPEND CMAKE_REQUIRED_LIBRARIES "${COMPILER_RT_BUILTINS_LIBRARY}")
   elseif (COMPILER_RT_HAS_GCC_S_LIB)
     list(APPEND CMAKE_REQUIRED_LIBRARIES gcc_s)
   elseif (COMPILER_RT_HAS_GCC_LIB)
     list(APPEND CMAKE_REQUIRED_LIBRARIES gcc)
   endif ()
 endif ()