[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@247245 91177308-0d34-0410-b5e6-96231b3b80d8

lib/MC/SubtargetFeature.cpp

@@ -56,7 +56,7 @@ static inline bool isEnabled(StringRef Feature) {
 ///
 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());
 }