Change getStringRepresentation to skip defaults.

I have a pending change for clang to use getStringRepresentation to check
that its DataLayout is in sync with llvm's.

getStringRepresentation is not called from llvm itself, so far it is mostly
a debugging aid, so the shorter strings are an independent improvement.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197740 91177308-0d34-0410-b5e6-96231b3b80d8