Fix some DenseMap use-after-rehash bugs and hoist MethodVFTableLocation

This re-lands r328845 with fixes for crbug.com/827810.

The initial motiviation was to hoist MethodVFTableLocation to global
scope so it could be forward declared.

In this patch, I noticed that MicrosoftVTableContext uses some risky
patterns. It has methods that return references to data stored in
DenseMaps. I've made some of them return by value for trivial structs
and I've moved some things into separate allocations.

llvm-svn: 329007