[SymbolFilePDB] Add support for resolving variable symbols

Summary:
Implement FindGlobalVariables and ParseVariableContext methods.

Compile unit information is necessary for resolving variable context, however some PDB symbols do not have this information. For now an empty DWARFExpression is used to construct a lldb::Variable instance with the limitation that using lldb to lookup the value of a global or local variable is not available.

This commit may slow down lit/SymbolFile/PDB/compilands.test since the test includes MS specific modules that spend more time parsing variables.

Reviewers: rnk, zturner, lldb-commits

Subscribers: aprantl, JDevlieghere, llvm-commits

Differential Revision: https://reviews.llvm.org/D45224

llvm-svn: 333049