[analyzer] PCH deserialization optimization.

We should not deserialize unused declarations from the PCH file. Achieve
this by storing the top level declarations during parsing
(HandleTopLevelDecl ASTConsumer callback) and analyzing/building a call
graph only for those.

Tested the patch on a sample ObjC file that uses PCH. With the patch, 
 the analyzes is 17.5% faster and clang consumes 40% less memory.
Got about 10% overall build/analyzes time decrease on a large Objective
C project.

A bit of CallGraph refactoring/cleanup as well..

llvm-svn: 154625