Retrieve a function PDB symbol correctly from nested blocks

Summary:
This patch fixes a problem with retrieving a function symbol by an
address in a nested block. In the current implementation of
ResolveSymbolContext function it retrieves a symbol with
PDB_SymType::None and then checks if found symbol's tag equals to
PDB_SymType::Function. So, if nested block's symbol was found,
ResolveSymbolContext does not resolve a function.

Reviewers: asmith, labath, zturner

Reviewed By: asmith, labath

Differential Revision: https://reviews.llvm.org/D47939
Patch by Aleksandr Urakov <aleksandr.urakov@jetbrains.com>

llvm-svn: 335822