Allow case-insensitive values for -march for AArch64 target 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 the AArch64 target.

llvm-svn: 240019
diff --git a/clang/lib/Driver/Tools.cpp b/clang/lib/Driver/Tools.cpp
index cef67d4..c8a1019 100644
--- a/clang/lib/Driver/Tools.cpp
+++ b/clang/lib/Driver/Tools.cpp
@@ -1848,7 +1848,8 @@
 getAArch64ArchFeaturesFromMarch(const Driver &D, StringRef March,
                                 const ArgList &Args,
                                 std::vector<const char *> &Features) {
-  std::pair<StringRef, StringRef> Split = March.split("+");
+  std::string MarchLowerCase = March.lower();
+  std::pair<StringRef, StringRef> Split = StringRef(MarchLowerCase).split("+");
 
   if (Split.first == "armv8-a" ||
       Split.first == "armv8a") {