Make march/target-cpu print a note with the list of valid values for ARM

When rejecting a march= or target-cpu command line parameter,
the message is quite lacking. This patch adds a note that prints
all possible values for the current target, if the target supports it.

This adds support for the ARM/AArch64 targets (more to come!). 

Differential Revision: https://reviews.llvm.org/D42978 

llvm-svn: 324673
diff --git a/clang/lib/Basic/Targets/ARM.h b/clang/lib/Basic/Targets/ARM.h
index fb0e7e6..af197fd 100644
--- a/clang/lib/Basic/Targets/ARM.h
+++ b/clang/lib/Basic/Targets/ARM.h
@@ -122,6 +122,8 @@
   bool hasFeature(StringRef Feature) const override;
 
   bool isValidCPUName(StringRef Name) const override;
+  void fillValidCPUList(SmallVectorImpl<StringRef> &Values) const override;
+
   bool setCPU(const std::string &Name) override;
 
   bool setFPMath(StringRef Name) override;