Revert "Allow case-insensitive values for -mcpu for ARM and AArch64"

This reverts commit r236859, as it broke multiple builds. I'll investigate
and reapply when safe.

llvm-svn: 236869
diff --git a/clang/lib/Driver/ToolChains.cpp b/clang/lib/Driver/ToolChains.cpp
index 151b51d..20a308d 100644
--- a/clang/lib/Driver/ToolChains.cpp
+++ b/clang/lib/Driver/ToolChains.cpp
@@ -126,7 +126,7 @@
 }
 
 static const char *GetArmArchForMCpu(StringRef Value) {
-  return llvm::StringSwitch<const char *>(Value.lower())
+  return llvm::StringSwitch<const char *>(Value)
     .Cases("arm9e", "arm946e-s", "arm966e-s", "arm968e-s", "arm926ej-s","armv5")
     .Cases("arm10e", "arm10tdmi", "armv5")
     .Cases("arm1020t", "arm1020e", "arm1022e", "arm1026ej-s", "armv5")