[CMake][Fuchsia] Enable assertions

[mcgrathr]

Summary

Enable assertions in both stages.
Release+Asserts is fast enough.
No need to let insanity through.

[phosek]

LGTM

cmake/caches/Fuchsia-stage2.cmake

if(NOT APPLE)
   set(LLVM_ENABLE_LLD ON CACHE BOOL "")
   set(CLANG_DEFAULT_LINKER lld CACHE STRING "")
 endif()
 
 if(APPLE)
   set(LLDB_CODESIGN_IDENTITY "" CACHE STRING "")
 endif()
 
+set(LLVM_ENABLE_ASSERTIONS ON CACHE BOOL "")
 set(CMAKE_BUILD_TYPE RelWithDebInfo CACHE STRING "")
 set(CMAKE_C_FLAGS_RELWITHDEBINFO "-O3 -gline-tables-only -DNDEBUG" CACHE STRING "")
 set(CMAKE_CXX_FLAGS_RELWITHDEBINFO "-O3 -gline-tables-only -DNDEBUG" CACHE STRING "")
 
 set(LLVM_BUILTIN_TARGETS "default;x86_64-fuchsia;aarch64-fuchsia" CACHE STRING "")
 foreach(target x86_64;aarch64)
   set(BUILTINS_${target}-fuchsia_CMAKE_SYSROOT ${FUCHSIA_${target}_SYSROOT} CACHE PATH "")
   set(BUILTINS_${target}-fuchsia_CMAKE_SYSTEM_NAME Fuchsia CACHE STRING "")

cmake/caches/Fuchsia.cmake

 # This file sets up a CMakeCache for a Fuchsia toolchain build.
 
 set(LLVM_TARGETS_TO_BUILD Native CACHE STRING "")
 
 set(PACKAGE_VENDOR Fuchsia CACHE STRING "")
 
 set(LLVM_INCLUDE_EXAMPLES OFF CACHE BOOL "")
 set(LLVM_INCLUDE_TESTS OFF CACHE BOOL "")
 set(LLVM_INCLUDE_DOCS OFF CACHE BOOL "")
 set(CLANG_INCLUDE_TESTS OFF CACHE BOOL "")
 set(LLVM_ENABLE_BACKTRACES OFF CACHE BOOL "")
 set(LLVM_ENABLE_TERMINFO OFF CACHE BOOL "")
 set(LLVM_ENABLE_ZLIB OFF CACHE BOOL "")
 set(CLANG_PLUGIN_SUPPORT OFF CACHE BOOL "")
 
+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()
 
 if(APPLE)