[NativePDB] Decouple AST reconstruction from lldb Symbol creation.

Previously the code that parsed debug info to create lldb's Symbol
objects such as Variable, Type, Function, etc was tightly coupled
to the AST reconstruction code.  This made it difficult / impossible
to implement functions such as ParseDeclsForContext() that were only
supposed to be operating on clang AST's.  By splitting these apart,
the logic becomes much cleaner and we have a clear separation of
responsibilities.

llvm-svn: 349383