Use it to calculate UserLabelPrefix, instead of specifying it (often
incorrectly).

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@262737 91177308-0d34-0410-b5e6-96231b3b80d8