[clang][Fuchsia] Allow for setting the default build type

[leonardchan]

Summary

Prior to this, CMAKE_BUILD_TYPE would unconditionally be Release. This allows me to add -DCMAKE_BUILD_TYPE=Debug to override this rather than manually changing the cache file.

[phosek]

This shouldn't be needed, you can do:

cmake ... -DCMAKE_BUILD_TYPE=Debug -C path/to/Fuchsia-stage2.cmake

This will override the value inside the cache file.

clang/cmake/caches/Fuchsia-stage2.cmake

 set(CLANG_DEFAULT_UNWINDLIB libunwind CACHE STRING "")
 set(CLANG_ENABLE_ARCMT OFF CACHE BOOL "")
 set(CLANG_ENABLE_STATIC_ANALYZER ON CACHE BOOL "")
 set(CLANG_PLUGIN_SUPPORT OFF CACHE BOOL "")
 
 set(ENABLE_LINKER_BUILD_ID ON CACHE BOOL "")
 set(ENABLE_X86_RELAX_RELOCATIONS ON CACHE BOOL "")
 
+if (NOT DEFINED CMAKE_BUILD_TYPE)
   set(CMAKE_BUILD_TYPE Release CACHE STRING "")
+endif()
+
 if (APPLE)
   set(CMAKE_OSX_DEPLOYMENT_TARGET "10.13" CACHE STRING "")
 elseif(WIN32)
   set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreaded" CACHE STRING "")
 endif()
 
 if(APPLE)
   list(APPEND BUILTIN_TARGETS "default")