Move computing the base subobject info for a class into the...

Move computing the base subobject info for a class into the RecordLayoutBuilder because we're going to need it to be able to correctly add offsets for primary virtual bases.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@105102 91177308-0d34-0410-b5e6-96231b3b80d8