[Parser] Don't crash on MS assembly if target desc/asm parser isn't linked in.

Summary:
Instead, emit a diagnostic and return an empty ASM node, as we do if the target
is missing.

Filter this diagnostic out in clangd, where it's not meaningful.

Fixes https://github.com/clangd/clangd/issues/222

Reviewers: kadircet

Subscribers: mgorny, ilya-biryukov, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D71189