[NativePDB] Add basic support of methods recostruction in AST

Summary:
This patch adds the basic support of methods reconstruction by native PDB
plugin. It contains only most obvious changes (it processes LF_ONEMETHOD and
LF_METHOD records), some things still remain unsolved:

- mangled names retrieving;
- support of template methods.

Reviewers: zturner, labath, lemo, stella.stamenova

Reviewed by: zturner

Differential Revision: https://reviews.llvm.org/D56126

llvm-svn: 352464