print-supported-cpus quality of life patch.

Claim all input files so that clang does not give a warning. Add two
short-cut aliases: -mcpu=? and -mtune=?.

llvm-svn: 364362
diff --git a/clang/docs/CommandGuide/clang.rst b/clang/docs/CommandGuide/clang.rst
index f511022..7b08736 100644
--- a/clang/docs/CommandGuide/clang.rst
+++ b/clang/docs/CommandGuide/clang.rst
@@ -330,6 +330,10 @@
   through --target=<architecture> or -arch <architecture>). If no target is
   specified, the system default target will be used.
 
+.. option:: -mcpu=?, -mtune=?
+
+  Aliases of --print-supported-cpus
+
 .. option:: -march=<cpu>
 
   Specify that Clang should generate code for a specific processor family