Add tests for -language-mode flag

Author: dempseyatgithub

test/Driver/swift-version.swift

@@ -12,6 +12,9 @@
 // RUN: not %target-swiftc_driver -swift-version 4 -typecheck %s 2>&1 | %FileCheck --check-prefix ERROR_4 %s
 // RUN: not %target-swiftc_driver -swift-version 5 -typecheck %s 2>&1 | %FileCheck --check-prefix ERROR_5 %s
 
+// RUN: not %target-swiftc_driver -language-mode 4 -typecheck %s 2>&1 | %FileCheck --check-prefix ERROR_4 %s
+// RUN: not %target-swiftc_driver -language-mode 5 -typecheck %s 2>&1 | %FileCheck --check-prefix ERROR_5 %s
+
 // BAD: invalid value
 // BAD: note: valid arguments to '-swift-version' are '4', '4.2', '5'