Include insertion gone for most symbols in static/auto index

[sam-mccall]

This is a regression:

• caused by https://reviews.llvm.org/rL358571 (require header guards)
• exacerbated by https://reviews.llvm.org/rL358605 (caches the header-guard check)

The theory is that because symbols are first declared in the file, and the header-guard controlling macro isn't identified until the end of the file.

When indexing a preamble post-hoc this shouldn't matter (PP/HeaderSearchInfo has all the information by the time we process the decls), but the static/auto index doesn't use a preamble and processes decls as they are found.

[sam-mccall]

Fixed in r359880