Make TargetInfo store an actual DataLayout instead of a string.
Use it to calculate UserLabelPrefix, instead of specifying it (often
Note that the *actual* user label prefix has always come from the
DataLayout, and is handled within LLVM. The main thing clang's
TargetInfo::UserLabelPrefix did was to set the #define value. Having
these be different from each-other is just silly.
Differential Revision: http://reviews.llvm.org/D17183