[Preamble] Allow recursive inclusion of header-guarded mainfile.

Summary:
This is guaranteed to be a no-op without the preamble, so should be a
no-op with it too.

Partially fixes https://github.com/clangd/clangd/issues/337
This doesn't yet work for #ifndef guards, which are not recognized in preambles.
see D78038

I can't for the life of me work out how to test this outside clangd.
The original reentrant preamble diagnostic was untested, I added a test
to clangd for that too.

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D78366