[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