Expose ValueMap's mutex type as a typedef instead of a sys::Mutex.

Expose ValueMap's mutex type as a typedef instead of a sys::Mutex.

This enables static polymorphism of the mutex type, which is
necessary in order to replace the standard mutex implementation
with a different type.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211080 91177308-0d34-0410-b5e6-96231b3b80d8