fix arguments testcase to handle specific error

Author: Nonchalant

test/Frontend/unknown-arguments.swift

@@ -10,5 +10,5 @@
 // RUN: not %swiftc_driver -D Correct -DAlsoCorrect -D@#%! -D Swift=Cool -D-D -c %s -o %t.o 2>&1 | %FileCheck -check-prefix=INVALID-COND %s
 // INVALID-COND: <unknown>:0: error: conditional compilation flags must be valid Swift identifiers (rather than '@#%!')
 // INVALID-COND-NEXT: <unknown>:0: warning: conditional compilation flags do not have values in Swift; they are either present or absent (rather than 'Swift=Cool')
-// INVALID-COND-NEXT: <unknown>:0: error: conditional compilation flags must be valid Swift identifiers (rather than '-D')
+// INVALID-COND-NEXT: <unknown>:0: error: invalid argument '-D-D'; did you provide a redundant '-D' in your build settings?