Merge r341697 into the 7.0 branch : warn_stdlibcxx_not_found: suggest '-stdlib=libc++' instead of '-std'

[tstellar]

Bugzilla Link	39708
Resolution	FIXED
Resolved on	Nov 19, 2018 16:36
Version	7.0
OS	All
Blocks	#38454
Fixed by commit(s)	r341697 r347276

Extended Description

Is it OK to merge the following revision(s) to the 7.0 branch?

[tstellar]

https://reviews.llvm.org/rL341697

[tstellar]

Merged: r347276