[clang] [Clang] fix crash in codegen caused by deferred asm diagnostics under -fopenmp (PR #147163)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

We shouldn't have invalid decls unless we've emitted an error.

If we're making some decision like "this function is an error if it's used as a host function", we should probably be marking the FunctionDecl explicitly with that decision, and CodeGen shouldn't try to emit such a function.

Also, the decision whether a function counts as "used" should probably be driven by Sema.  Currently you don't get errors at all with -fsyntax-only.

https://github.com/llvm/llvm-project/pull/147163