CodeGen: Clean up implementation of vtable initializer builder. NFC.

CodeGen: Clean up implementation of vtable initializer builder. NFC.

- Simplify signature of CreateVTableInitializer function.
- Move vtable component builder to a separate function.
- Remove unnecessary accessors from VTableLayout class.

This is in preparation for a future change that will alter the type of the
vtable initializer.

Differential Revision: https://reviews.llvm.org/D22642

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@280897 91177308-0d34-0410-b5e6-96231b3b80d8