Make nrvo-string test more robust.

This is a follow-up to r364466, but better implemented. Original
commit message still applies:

    The breakpoint locations were in places where clang doesn't actually
    emit a source location for and depend on the debugger's ability to
    move the breakpoint forward onto a line that is already in the
    function epilogue. In my testing older versions of LLDB fail to do
    that, so I'm modifying the test to force a break-able location by
    calling a noinline function.

    <rdar://problem/52079841>

llvm-svn: 364589