Get rid of VTableContext::ComputeMethodVTableIndices() and VTableContext::getNumVirtualFunctionPointers(); also add some tests for the VTable indices output to make sure we don't regress

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@183298 91177308-0d34-0410-b5e6-96231b3b80d8