[clangd] Support include canonicalization in symbol leve.

Summary:
Symbols with different canonical includes might be defined in the same header
(e.g. symbols defined in STL <iosfwd>). This patch adds support for mapping from
qualified symbol names to canonical headers and special mapping for symbols in <iosfwd>

Reviewers: sammccall, hokein

Reviewed By: sammccall

Subscribers: klimek, ilya-biryukov, jkorous-apple, cfe-commits

Differential Revision: https://reviews.llvm.org/D43869

llvm-svn: 326456