[DebugInfoPDB] Raise getSymIndexId() up to PDBSymbol

Every symbol, no matter what it's tag is, supports the method
getSymIndexId().  However, this was being forwarded on every
concrete symbol type, so if someone had a PDBSymbol that they
didn't know what type it was (or simply didn't have an instance
of the concrete symbol type), they would not be able to get its
index id.  This patch moves the method up to PDBSymbol, so that
no matter what type of object you have, you can always get its
id.

llvm-svn: 261153