[CUDA] When compilation fails, print the compilation mode.

Summary:
That is, instead of "1 error generated", we now say "1 error generated
when compiling for sm_35".

This (partially) solves a usability foogtun wherein e.g. users call a
function that's only defined on sm_60 when compiling for sm_35, and they
get an unhelpful error message.

Reviewers: tra

Subscribers: sanjoy, cfe-commits

Differential Revision: https://reviews.llvm.org/D37548

llvm-svn: 312736