Extend FindTypes with CompilerContext to allow filtering by language.
This patch is also motivated by the Swift branch and is effectively NFC for the single-TypeSystem llvm.org branch.
In multi-language projects it is extremely common to have, e.g., a
Clang type and a similarly-named rendition of that same type in
another language. When searching for a type It is much cheaper to pass
a set of supported languages to the SymbolFile than having it
materialize every result and then rejecting the materialized types
that have the wrong language.
Differential Revision: https://reviews.llvm.org/D66546