InstrProf: Change the extension of the default profile

Change the name of the default profile dumped by compiler-rt to
default.profraw.  This distinguishes it more clearly from the
(incompatible) format output by llvm-profdata that is read by clang
-fprofile-instr-use.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@204676 91177308-0d34-0410-b5e6-96231b3b80d8