Fix bug in SourceManager::getDecomposedInstantiationLocSlowCase.

It would add up relative (decomposed) offsets like in getDecomposedSpellingLocSlowCase, but while
it makes sense to preserve the offset among lexed spelling locations, it doesn't make
sense to add anything to the offset of the instantiation location. The instantiation
location will be the same regardless of the relative offset in the tokens that were
instantiated.

This bug didn't actually affect anything because, currently, in practice we never create macro
locations with relative offset greater than 0.

llvm-svn: 134586