Add tests for all DescriptionString in Targets.cpp.

These right now just test that the same string is present in two files, but will
become more useful as clang's handling of DataLayout is refactored.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197347 91177308-0d34-0410-b5e6-96231b3b80d8