[clangd] Make FileIndex aware of the main file

Summary:
It was previously only indexing the preamble decls. The new
implementation will index both the preamble and the main AST and
report both sets of symbols, preferring the ones from the main AST
whenever the symbol is present in both.
The symbols in the main AST slab always store all information
available in the preamble symbols, possibly adding more,
e.g. definition locations.

Reviewers: hokein, ioeric

Reviewed By: ioeric

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

Differential Revision: https://reviews.llvm.org/D50889

llvm-svn: 340404