Replace std::string::find == 0 with StringRef::startswith

This is both more readable and faster. Found by clang-tidy's
abseil-string-find-startswith.
diff --git a/clang/lib/Driver/ToolChains/Arch/AArch64.cpp b/clang/lib/Driver/ToolChains/Arch/AArch64.cpp
index dc31a5a..b21cfac 100644
--- a/clang/lib/Driver/ToolChains/Arch/AArch64.cpp
+++ b/clang/lib/Driver/ToolChains/Arch/AArch64.cpp
@@ -140,7 +140,8 @@
   // Handle CPU name is 'native'.
   if (MtuneLowerCase == "native")
     MtuneLowerCase = std::string(llvm::sys::getHostCPUName());
-  if (MtuneLowerCase == "cyclone" || MtuneLowerCase.find("apple") == 0) {
+  if (MtuneLowerCase == "cyclone" ||
+      StringRef(MtuneLowerCase).startswith("apple")) {
     Features.push_back("+zcm");
     Features.push_back("+zcz");
   }