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/ToolChain.cpp b/clang/lib/Driver/ToolChain.cpp
index f7b7402..82eb854 100644
--- a/clang/lib/Driver/ToolChain.cpp
+++ b/clang/lib/Driver/ToolChain.cpp
@@ -303,7 +303,7 @@
     // Thumb2 is the default for V7 on Darwin.
     //
     // FIXME: Thumb should just be another -target-feaure, not in the triple.
-    StringRef CPU = Triple.isOSBinFormatMachO()
+    std::string CPU = Triple.isOSBinFormatMachO()
       ? tools::arm::getARMCPUForMArch(Args, Triple)
       : tools::arm::getARMTargetCPU(Args, Triple);
     StringRef Suffix =