[clangd] Fix code completion of macros defined in the preamble region of the main file.

Summary:
This is a tricky case (we baked the assumption that symbols come from
the preamble xor mainfile pretty deeply) and the fix is a bit of a hack:
We look at the code to guess the macro names, and deserialize them from
the preamble "by hand".

Reviewers: ilya-biryukov

Subscribers: ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60937

llvm-svn: 359778