[libcxx] Optimize away unneeded length calculation in basic_string::compare(const char*)

Summary:
This patch optimizes basic_string::compare to use strcmp when the default char_traits has been given.
See PR19900 for more information. https://llvm.org/bugs/show_bug.cgi?id=19900

Reviewers: mclow.lists

Subscribers: bkramer, cfe-commits

Differential Revision: http://reviews.llvm.org/D12355

llvm-svn: 246266