[CMake][Fuchsia] Disable in-process cc1

The in-process cc1 currently breaks Goma client and therefore prevents
the use of tip-of-tree Clang with Goma, so we disable the in-process cc1
in our toolchain until this issue is resolved.

clang/cmake/caches/Fuchsia-stage2.cmake

@@ -24,6 +24,7 @@ if(NOT APPLE)
   set(CLANG_DEFAULT_OBJCOPY llvm-objcopy CACHE STRING "")
 endif()
 set(CLANG_DEFAULT_RTLIB compiler-rt CACHE STRING "")
+set(CLANG_SPAWN_CC1 ON CACHE BOOL "")
 set(CLANG_ENABLE_ARCMT OFF CACHE BOOL "")
 set(CLANG_ENABLE_STATIC_ANALYZER ON CACHE BOOL "")
 set(CLANG_PLUGIN_SUPPORT OFF CACHE BOOL "")