[PDB] Restore AST from PDB symbols

Summary:
This patch adds an implementation of retrieving of declarations and declaration
contexts based on PDB symbols.

PDB has different type symbols for const-qualified types, and this
implementation ensures that only one declaration was created for both const
and non-const types, but creates different compiler types for them.

The implementation also processes the case when there are two symbols
corresponding to a variable. It's possible e.g. for class static variables,
they has one global symbol and one symbol belonging to a class.

PDB has no info about namespaces, so this implementation parses the full symbol
name and tries to figure out if the symbol belongs to namespace or not,
and then creates nested namespaces if necessary.

Reviewers: asmith, zturner, labath

Reviewed By: asmith

Subscribers: aleksandr.urakov, teemperor, lldb-commits, stella.stamenova

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D51162

llvm-svn: 341782