Allow copy elision in path concatenation

Summary:
Just port of libstdc++'s fix to libc++ fs: https://github.com/gcc-mirror/gcc/commit/e6ac4004fe49d785c63bf87aec4b095b5ce1d19f

Author of fix: Jonathan Wakely

Reviewers: EricWF, mclow.lists

Reviewed By: EricWF

Subscribers: smeenai, christof, cfe-commits, llvm-commits

Differential Revision: https://reviews.llvm.org/D46593

llvm-svn: 331910