[Offload] Fix the offload cache file triggering libc++ / libstdc++ mixing (#126313)

Summary:
We originally wanted `-stdlib=libc++` by default so that it could use
offloading support in libc++, however this causes issues with out the
Offloading proejct itself is built. Is the user builds the LLVM libs
with libstdc++ then uses this cache it will enable this option by
default for the ensuing build of the offloading libraries with the newly
build clang. This will cause a lot of linker failured because the C++
library doesn't match.

Long term I think the proper solution to this is to make better use of
clang configuration files, but I don't know a good way to do that by
default. For now just make it build right.

Author: jhuber6

offload/cmake/caches/Offload.cmake

@@ -2,9 +2,6 @@ set(LLVM_ENABLE_PROJECTS "clang;clang-tools-extra;lld" CACHE STRING "")
 set(LLVM_ENABLE_RUNTIMES "compiler-rt;libunwind;libcxx;libcxxabi;openmp;offload" CACHE STRING "")
 set(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR ON CACHE BOOL "")
 
-set(CLANG_DEFAULT_CXX_STDLIB "libc++" CACHE STRING "")
-set(CLANG_DEFAULT_LINKER "lld" CACHE STRING "")
-
 set(LLVM_RUNTIME_TARGETS default;amdgcn-amd-amdhsa;nvptx64-nvidia-cuda CACHE STRING "") 
 set(RUNTIMES_nvptx64-nvidia-cuda_CACHE_FILES "${CMAKE_SOURCE_DIR}/../libcxx/cmake/caches/NVPTX.cmake" CACHE STRING "")
 set(RUNTIMES_amdgcn-amd-amdhsa_CACHE_FILES "${CMAKE_SOURCE_DIR}/../libcxx/cmake/caches/AMDGPU.cmake" CACHE STRING "")