[llvm-strings] Add support for the -a/--all options

They don't actually change nay behaviour, as llvm-strings currently
checks the whole object without looking at individual sections anyway.

This allows using llvm-strings in a context that explicitly passes
the -a option.

Differential Revision: https://reviews.llvm.org/D40020

llvm-svn: 318185
diff --git a/llvm/test/tools/llvm-strings/all-sections.test b/llvm/test/tools/llvm-strings/all-sections.test
new file mode 100644
index 0000000..dcf3519
--- /dev/null
+++ b/llvm/test/tools/llvm-strings/all-sections.test
@@ -0,0 +1,3 @@
+RUN: llvm-strings -a %S/Inputs/abcd | FileCheck %s
+RUN: llvm-strings --all %S/Inputs/abcd | FileCheck %s
+CHECK: abcd