Hoist MethodVFTableLocation out of MicrosoftVTableContext, NFC

This allows forward declaring it so that we can add it to
MicrosoftMangleContext::mangleVirtualMemPtrThunk without including
VTableBuilder.h. That saves a hashtable lookup when emitting virtual
member pointer functions.

It also shortens a really long type name. This struct has "VFtable" in
the name, so it seems pretty unlikely that someone will assume it is
generally useful for non-MS C++ ABI stuff.

llvm-svn: 328845