Downgrade reserved module identifier error into a warning

Any project that wants to import std; potentially needs to be able to
build a module that does export std;. We silenced the error diagnostic
if the module identified itself as a system header, but this isn't
quite good enough, what we really need is a way to identify a system
module. It would be nice for that feature to be shared among the major
implementations, so this downgrades the diagnostic from an error to a
warning temporarily to give implementers time to determine what that
mechanism will look like. We may convert this warning back into an
error in a future release of Clang, but it's not guaranteed we will do
so.

Fixes https://github.com/llvm/llvm-project/issues/61446
Differential Revision: https://reviews.llvm.org/D146986