Allow case-insensitive values for -march for ARM in line with GCC.

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

llvm-svn: 239527
diff --git a/clang/lib/Driver/Tools.h b/clang/lib/Driver/Tools.h
index 133a389..0420eea 100644
--- a/clang/lib/Driver/Tools.h
+++ b/clang/lib/Driver/Tools.h
@@ -227,8 +227,8 @@
 namespace arm {
   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 std::string getARMArch(const llvm::opt::ArgList &Args,
+                               const llvm::Triple &Triple);
   const char* getARMCPUForMArch(const llvm::opt::ArgList &Args,
                                 const llvm::Triple &Triple);
   const char* getLLVMArchSuffixForARM(StringRef CPU, StringRef Arch);