Fix r359229 which tried to fix r359159...
Fix r359229 which tried to fix r359159...

When r359229 added noexcept to the declaration of `~mutex`, it didn't
add it to the definition which caused -Wimplicit-exception-spec-mismatch
to fire. This just adapts the definition to agree with the declaration.

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@359275 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx