[llvm-ar] Document response file support in --help

Also a test for this.

Differential Revision: https://reviews.llvm.org/D63836

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@364673 91177308-0d34-0410-b5e6-96231b3b80d8

test/tools/llvm-ar/response-file.test

@@ -0,0 +1,9 @@
+RUN: touch %t-a.txt %t-b.txt %t-c.txt
+RUN: echo "rcs %t.a %t-a.txt %t-b.txt %t-c.txt" > %t-response
+RUN: rm -f %t.a
+RUN: llvm-ar @%t-response
+RUN: llvm-ar t %t.a | FileCheck %s
+
+CHECK:      a.txt
+CHECK-NEXT: b.txt
+CHECK-NEXT: c.txt