Mark __clear_and_shrink() as noexcept. This prevents the generation of a catch...

Mark __clear_and_shrink() as noexcept. This prevents the generation of a catch block and call to terminate in string's move assignment. Thanks to Howard for the 'catch'.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@333435 91177308-0d34-0410-b5e6-96231b3b80d8