[Codeview/PDB] Rename some methods for clarity.

NFC, this just renames some methods to better express what they
do, and also adds a few helper methods to add some symmetry to the
API in a few places (for example there was a getStringFromId but not
a getIdFromString method in the string table).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@328221 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed