Allow case-insensitive values for -mcpu for ARM

GCC allows case-insensitive values for -mcpu, -march and -mtune options.
This patch implements the same behaviour for the -mcpu option.

llvm-svn: 239059
diff --git a/clang/lib/Driver/Tools.h b/clang/lib/Driver/Tools.h
index 2ecaf66..1d51552 100644
--- a/clang/lib/Driver/Tools.h
+++ b/clang/lib/Driver/Tools.h
@@ -224,8 +224,8 @@
 } // end namespace hexagon.
 
 namespace arm {
-  StringRef getARMTargetCPU(const llvm::opt::ArgList &Args,
-                            const llvm::Triple &Triple);
+  std::string getARMTargetCPU(const llvm::opt::ArgList &Args,
+                              const llvm::Triple &Triple);
   const StringRef getARMArch(const llvm::opt::ArgList &Args,
                              const llvm::Triple &Triple);
   const char* getARMCPUForMArch(const llvm::opt::ArgList &Args,