[clangd] Fix header-guard check for include insertion, and don't index header guards.

Summary:
Both of these attempt to check whether a header guard exists while parsing the
file. However the file is only marked as guarded once clang finishes processing
it. We defer the checks and work until SymbolCollector::finish().

This is ugly and ad-hoc, deferring *all* work might be cleaner.

Reviewers: kadircet

Subscribers: ilya-biryukov, MaskRay, jkorous, mgrang, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61442

llvm-svn: 359880