Merging r341697:

------------------------------------------------------------------------
r341697 | arphaman | 2018-09-07 11:59:45 -0700 (Fri, 07 Sep 2018) | 4 lines

warn_stdlibcxx_not_found: suggest '-stdlib=libc++' instead of '-std'

Addresses first post-commit feedback for r335081 from Nico

------------------------------------------------------------------------

llvm-svn: 347276