[CodeView] Add prefix to CodeView registers.

Adds CVReg to CodeView register names to prevent a duplicate symbol with
CR3 defined in termios.h, as suggested by Zachary on the mailing list.

http://lists.llvm.org/pipermail/llvm-dev/2018-May/123372.html

Differential revision: https://reviews.llvm.org/D47478

rdar://39863705

llvm-svn: 333421