This commit is a combination of following changes:
(1) Cache PDB's global scope(executable) in SymbolFilePDB
(2) Change naming of `cu` to `compiland` which is PDB specific
(3) 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
(4) Add GetPDBCompilandByUID method to simply code
(5) 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.
Also the previous implementation intended to call
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.