[lldb] Add support to cache a PDB's global scope and fix a bug in getting the...

[lldb] Add support to cache a PDB's global scope and fix a bug in getting the source file name for a compiland

Summary:
This commit is a combination of the following changes:
    
- Cache PDB's global scope (executable) in SymbolFilePDB  

- Change naming of `cu` to `compiland` which is PDB specific   

- Change ParseCompileUnitForSymIndex to ParseCompileUnitForUID.
   Prefer using a common name `UID` instead of PDB's `System Index`
   Adding one more argument `index` to this method, which is used to
   specify the index of the compile unit in a cached compile unit array
    
- Add GetPDBCompilandByUID method to simply code
    
- Fix a bug in getting the source file name for a PDB compiland.
   For some reason, PDBSymbolCompiland::getSourceFileName() could
   return an empty name, so if that is true, we have to walk through all
   source files of this compiland and determine the right source file
   used to generate this compiland based on language indicated.
    
   The previous implementation called PDBSession::findOneSourceFile 
   method to get its name for the compiland. This is not accurate since
   the `one source file` found could be a header other than source file.

Reviewers: zturner, lldb-commits

Reviewed By: zturner

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D41428

llvm-svn: 322433