[refactor][rename] Use a single base class for class that finds

a declaration at location and for class that searches for all occurrences of
a specific declaration

This commit uses a single RecursiveSymbolVisitor class for both
USRLocFindingASTVisitor and NamedDeclOccurrenceFindingVisitor to avoid duplicate
traversal code. It also traverses nested name specifier locs in the new class
and remove the separate matching step.

Differential Revision: https://reviews.llvm.org/D34949

llvm-svn: 307898