[PDB] Resolve a symbol context block info correctly

Summary:
This patch allows to resolve a symbol context block info even if a function
info was not requested. Also it adds the correct resolving of nested blocks
(the previous implementation used function blocks instead of them).

Reviewers: zturner, asmith, labath

Reviewed By: asmith

Subscribers: lldb-commits, stella.stamenova

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D51104

llvm-svn: 340901