[test] Specify an explicit darwin version in a triple in

`test/Driver/debug-options.c` to ensure that the driver
selects the DWARF 2 version as intended by the test.

Fixes the `test/Driver/debug-options.c` test regression on GreenDragon
on macOS that started failing after r357713.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@357740 91177308-0d34-0410-b5e6-96231b3b80d8

test/Driver/debug-options.c

@@ -25,7 +25,7 @@
 // RUN:             | FileCheck -check-prefix=G -check-prefix=G_DWARF4 %s
 
 // Darwin.
-// RUN: %clang -### -c -g %s -target x86_64-apple-darwin 2>&1 \
+// RUN: %clang -### -c -g %s -target x86_64-apple-darwin14 2>&1 \
 // RUN:             | FileCheck -check-prefix=G_STANDALONE \
 // RUN:                         -check-prefix=G_DWARF2 \
 // RUN:                         -check-prefix=G_LLDB %s