[test] Fix apparent typo in clang/test/Driver/std.c

Currently the test on line 3 is identical to the test on line 1.
Looking at the rest of the file (particularily the use of FOVERRIDE
as the check-prefix), I think it's pretty clear that this line
was supposed to use `-ftrigraphs` instead of `-trigraphs`.

Reviewed By: thakis

Differential Revision: https://reviews.llvm.org/D97796