[NativePDB] Add anonymous namespaces support

Summary:
This patch adds anonymous namespaces support to the native PDB plugin.

I had to reference from the main function variables of the types that are inside
of the anonymous namespace to include them in debug info. Without the references
they are not included. I think it's because they are static, then are visible
only in the current translation unit, so they are not needed without any
references to them.

There is also the problem case with variables of types that are nested in
template structs. For now I've left FIXME in the test because this case is not
related to the change.

Reviewers: zturner, asmith, labath, stella.stamenova, amccarth

Reviewed By: amccarth

Subscribers: zloyrobot, aprantl, teemperor, lldb-commits, leonid.mashinskiy

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D60817

llvm-svn: 358873