[OMPT] Rename ompt_mutex_impl_t to kmp_mutex_impl

The defintion is not part of the spec and thus should not have the prefix
"ompt_" but rather a prefix that indicates that this is implementation
specific.

Differential Revision: https://reviews.llvm.org/D41166

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@322621 91177308-0d34-0410-b5e6-96231b3b80d8