Override the datalayout in a module with -triple.

This matches llc's behavior.

Before this patch clang would create a TargetInfo base on -triple but a llvm
CodeGen based on the triple in the module.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197837 91177308-0d34-0410-b5e6-96231b3b80d8
Remove obsolete preamble FIXME

The MD5 is checked since r197755 which is as good as comparing buffer contents
in this context.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197842 91177308-0d34-0410-b5e6-96231b3b80d8