[CMake][Fuchsia] Use unchecked hardening mode for libc++

We set LLVM_ENABLE_ASSERTIONS in some builds which is now translated
to LIBCXX_HARDENING_MODE=safe, but our codebase is not yet clean and
we're getting assertion failures.

Author: petrhosek

clang/cmake/caches/Fuchsia-stage2.cmake

@@ -81,6 +81,7 @@ if(APPLE)
   set(LIBCXX_ABI_VERSION 2 CACHE STRING "")
   set(LIBCXX_ENABLE_SHARED OFF CACHE BOOL "")
   set(LIBCXX_ENABLE_STATIC_ABI_LIBRARY ON CACHE BOOL "")
+  set(LIBCXX_HARDENING_MODE "unchecked" CACHE STRING "")
   set(LIBCXX_USE_COMPILER_RT ON CACHE BOOL "")
   set(RUNTIMES_CMAKE_ARGS "-DCMAKE_OSX_DEPLOYMENT_TARGET=10.13;-DCMAKE_OSX_ARCHITECTURES=arm64|x86_64" CACHE STRING "")
 endif()

clang/cmake/caches/Fuchsia.cmake

@@ -116,6 +116,7 @@ else()
   set(LIBCXX_ABI_VERSION 2 CACHE STRING "")
   set(LIBCXX_ENABLE_SHARED OFF CACHE BOOL "")
   set(LIBCXX_ENABLE_STATIC_ABI_LIBRARY ON CACHE BOOL "")
+  set(LIBCXX_HARDENING_MODE "unchecked" CACHE STRING "")
   set(LIBCXX_USE_COMPILER_RT ON CACHE BOOL "")
   set(LLVM_ENABLE_RUNTIMES "compiler-rt;libcxx;libcxxabi;libunwind" CACHE STRING "")
   set(RUNTIMES_CMAKE_ARGS "-DCMAKE_OSX_DEPLOYMENT_TARGET=10.13;-DCMAKE_OSX_ARCHITECTURES=arm64|x86_64" CACHE STRING "")