[PDB] Improve performance of the PDB DIA plugin

Summary:
This patch improves performance of `SymbolFilePDB` on huge executables
in two ways:

- cache names of public symbols by address. When creating variables we are
  trying to get a mangled name for each one, and in `GetMangledForPDBData`
  we are enumerating all public symbols, which takes O(n) for each variable.
  With the cache we can retrieve a mangled name in O(log(n));

- cache section contributions. When parsing variables for context we are
  enumerating all variables and check if the current one is belonging
  to the current compiland. So we are retrieving a compiland ID
  for the variable. But in `PDBSymbolData::getCompilandId` for almost every
  variable we are enumerating all section contributions to check if the variable
  is belonging to it, and get a compiland ID from the section contribution
  if so. It takes O(n) for each variable, but with caching it takes about
  O(log(n)). I've placed the cache in `SymbolFilePDB` and have created
  `GetCompilandId` there. It actually duplicates `PDBSymbolData::getCompilandId`
  except for the cache part. Another option is to support caching
  in `PDBSymbolData::getCompilandId` and to place cache in `DIASession`, but it
  seems that the last one doesn't imply such functionality, because
  it's a lightweight wrapper over DIA and whole its state is only a COM pointer
  to the DIA session. Moreover, `PDBSymbolData::getCompilandId` is used only
  inside of `SymbolFilePDB`, so I think that it's not a bad place to do such
  things. With this patch `PDBSymbolData::getCompilandId` is not used at all.

This bottlenecks were found with profiling. I've discovered these on a simple
demo project of Unreal Engine (x86 executable ~72M, PDB ~82M).

This patch doesn't change external behavior of the plugin, so I think that
there's no need for additional testing (already existing tests should warn us
about regress, if any).

Reviewers: zturner, asmith, labath

Reviewed By: asmith

Subscribers: Hui, lldb-commits, stella.stamenova

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D53375

llvm-svn: 345013