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>