Decide on the expression language inside UserExpression

When the target settings are consulted to decide the expression language
is decided in CommandObjectExpression, this doesn't help if you're running
SBFrame::EvaluateExpression().  Moving the logic into UserExpression fixes
this.

Based on patch from scallanan@apple.com
Reviewed by: dawn
Subscribers: lldb-commits
Differential Revision: http://reviews.llvm.org/D13267

llvm-svn: 249624