[RISCV] Fix required features checking with empty string

In our downstream, we define some intrinsics that don't require any
extra extension enabled. Such as

TARGET_BUILTIN(__builtin_riscv_xxx, "LiLi", "nc", "")

But `split` function's `KeepEmpty` argument is True. Got the error message

error: builtin requires at least one of the following extensions support to be enabled : ''

when we use our customized intrinsic.

Reviewed By: craig.topper, wangpc

Differential Revision: https://reviews.llvm.org/D154596

Author: tclin914

clang/lib/Sema/SemaChecking.cpp

@@ -4481,7 +4481,7 @@ bool Sema::CheckRISCVBuiltinFunctionCall(const TargetInfo &TI,
   bool FeatureMissing = false;
   SmallVector<StringRef> ReqFeatures;
   StringRef Features = Context.BuiltinInfo.getRequiredFeatures(BuiltinID);
-  Features.split(ReqFeatures, ',');
+  Features.split(ReqFeatures, ',', -1, false);
 
   // Check if each required feature is included
   for (StringRef F : ReqFeatures) {