Apply constexpr to the mutex constructor. As a conforming extension, apply...

Apply constexpr to the mutex constructor.  As a conforming extension, apply constexpr to the condition_variable constructor.  These are important because it enables the compiler to construct these types at compile time, even though the object will be non-const.  Since they are constructed at compile time, there is no chance of a data race before they are constructed.

llvm-svn: 159901