[ADT] Switch a bunch of places in LLVM that were doing single-character
splits to actually use the single character split routine which does
less work, and in a debug build is *substantially* faster.

llvm-svn: 247245
diff --git a/llvm/lib/MC/SubtargetFeature.cpp b/llvm/lib/MC/SubtargetFeature.cpp
index 76574e9..b642f17 100644
--- a/llvm/lib/MC/SubtargetFeature.cpp
+++ b/llvm/lib/MC/SubtargetFeature.cpp
@@ -56,7 +56,7 @@
 ///
 static void Split(std::vector<std::string> &V, StringRef S) {
   SmallVector<StringRef, 3> Tmp;
-  S.split(Tmp, ",", -1, false /* KeepEmpty */);
+  S.split(Tmp, ',', -1, false /* KeepEmpty */);
   V.assign(Tmp.begin(), Tmp.end());
 }