[CMake] Use libc++ and compiler-rt for bootstrap Fuchsia Clang

[phosek]

Summary

We want to build the second stage compiler with libc++ and compiler-rt,
also include builtins and runtimes into extra bootstrap components to
ensure these get built.

cmake/caches/Fuchsia.cmake

 set(LLVM_ENABLE_ASSERTIONS ON CACHE BOOL "")
 set(CMAKE_BUILD_TYPE Release CACHE STRING "")
 
 set(BOOTSTRAP_LLVM_ENABLE_LTO ON CACHE BOOL "")
 if(NOT APPLE)
   set(BOOTSTRAP_LLVM_ENABLE_LLD ON CACHE BOOL "")
 endif()
 
+set(CLANG_DEFAULT_CXX_STDLIB libc++ CACHE STRING "")
+set(CLANG_DEFAULT_RTLIB compiler-rt CACHE STRING "")
+
 if(APPLE)
   set(COMPILER_RT_ENABLE_IOS OFF CACHE BOOL "")
   set(COMPILER_RT_ENABLE_TVOS OFF CACHE BOOL "")
   set(COMPILER_RT_ENABLE_WATCHOS OFF CACHE BOOL "")
 endif()
 
 set(CLANG_BOOTSTRAP_TARGETS
   check-all
foreach(variableName ${variableNames})
   if(variableName MATCHES "^STAGE2_")
     string(REPLACE "STAGE2_" "" new_name ${variableName})
     list(APPEND EXTRA_ARGS "-D${new_name}=${${variableName}}")
   endif()
 endforeach()
 
 # Setup the bootstrap build.
 set(CLANG_ENABLE_BOOTSTRAP ON CACHE BOOL "")
+set(CLANG_BOOTSTRAP_EXTRA_DEPS
+  builtins
+  runtimes
+  CACHE STRING "")
 set(CLANG_BOOTSTRAP_CMAKE_ARGS
   ${EXTRA_ARGS}
   -C ${CMAKE_CURRENT_LIST_DIR}/Fuchsia-stage2.cmake
   CACHE STRING "")