[Codeview/PDB] Rename some methods for clarity.

[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).

llvm-svn: 328221