[CodeView] Factor some code out of TypeTableBuilder.

This class had some code that would automatically remap type
indices before hashing and serializing.  The only caller of
this method was the TypeStreamMerger anyway, and the method
doesn't make general sense, and prevents making certain future
improvements to the class.  So, factoring this up one level
into the TypeStreamMerger where it belongs.

llvm-svn: 319377