[CMake] Add option to disable driver build in Fuchsia cache file

Linking the driver binary can take a long time, particularly with debug
info. This adds an option to disable the driver build meant for local use.

Differential Revision: https://reviews.llvm.org/D159130

clang/cmake/caches/Fuchsia-stage2.cmake

@@ -31,7 +31,10 @@ set(LLDB_ENABLE_LIBEDIT OFF CACHE BOOL "")
 
 if(WIN32)
   set(LLVM_USE_CRT_RELEASE "MT" CACHE STRING "")
-else()
+  set(FUCHSIA_DISABLE_DRIVER_BUILD ON)
+endif()
+
+if (NOT FUCHSIA_DISABLE_DRIVER_BUILD)
   set(LLVM_TOOL_LLVM_DRIVER_BUILD ON CACHE BOOL "")
   set(LLVM_DRIVER_TARGET llvm-driver)
 endif()