[PGO] Stop using invalid char in instr variable names.

Before the patch, -fprofile-instr-generate compile will fail
if no integrated-as is specified when the file contains
any static functions (the -S output is also invalid).

This patch fixed the issue. With the change, the index format
version will be bumped up by 1. Backward compatibility is 
preserved with this change.

Differential Revision: http://reviews.llvm.org/D15243

llvm-svn: 255365