[clangd] Factor out the heuristic resolver code into its own class

The patch also does some cleanup on the interface of the entry
points from TargetFinder into the heuristic resolution code.

Since the heuristic resolver is created in a place where the
ASTContext is available, it can store the ASTContext and the
NameFactory hack can be removed.

Differential revision: https://reviews.llvm.org/D92290