[CodeView] Add more DebugSubsection implementations.

This adds implementations for Symbols and FrameData, and renames
the existing codeview::StringTable class to conform to the
DebugSectionStringTable convention.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@304222 91177308-0d34-0410-b5e6-96231b3b80d8
23 files changed