[CMake][Fuchsia] Enable assertions and backtraces in stage 1 build

[abrachet]

I also found D120280 where we disabled assertions. Are you still of the opinion that we can enable these or does that remind you of whatever it was you found back then?

[phosek]

LGTM I'm fine enabling them for the first stage, but we should keep an eye on the build times after this lands.

clang/cmake/caches/Fuchsia.cmake

 set(CLANG_DEFAULT_UNWINDLIB libunwind CACHE STRING "")
 set(CLANG_ENABLE_ARCMT OFF CACHE BOOL "")
 set(CLANG_ENABLE_STATIC_ANALYZER OFF 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 "")
 
-set(LLVM_ENABLE_ASSERTIONS OFF CACHE BOOL "")
-set(LLVM_ENABLE_BACKTRACES OFF CACHE BOOL "")
+set(LLVM_ENABLE_ASSERTIONS ON CACHE BOOL "")
+set(LLVM_ENABLE_BACKTRACES ON CACHE BOOL "")
 set(CMAKE_BUILD_TYPE Release CACHE STRING "")
 if(APPLE)
   set(CMAKE_OSX_DEPLOYMENT_TARGET "10.13" CACHE STRING "")
 elseif(WIN32)
   set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreaded" CACHE STRING "")
 endif()
 
 if(APPLE)