MI support for '-var-create $regname' command.

Summary:
This command is currently processed using expression evaluation, meaning the variable binds to the result of the expression not the register.
Therefore any subsequent calls to '-var-assign' will not update the register. Fixed by detecting '$' prefix for registers according to specification.

Thanks, Ewan

Patch from ewan@codeplay.com

Reviewers: EwanCrawford

Subscribers: lldb-commits, deepak2427

Differential Revision: http://reviews.llvm.org/D8965

llvm-svn: 235195