[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

Author: DanielMcIntosh

clang/test/Driver/std.c

@@ -1,6 +1,6 @@
 // RUN: %clang -w -std=c99 -trigraphs -std=gnu99 %s -E -o - | FileCheck -check-prefix=OVERRIDE %s
 // OVERRIDE: ??(??)
-// RUN: %clang -w -std=c99 -trigraphs -std=gnu99 %s -E -o - | FileCheck -check-prefix=FOVERRIDE %s
+// RUN: %clang -w -std=c99 -ftrigraphs -std=gnu99 %s -E -o - | FileCheck -check-prefix=FOVERRIDE %s
 // FOVERRIDE: ??(??)
 // RUN: %clang -w -ansi %s -E -o - | FileCheck -check-prefix=ANSI %s
 // ANSI: []