[Driver] Use StringRef::substr instead of StringRef::slice (NFC) (llvm#139455)

StringRef::substr is shorter here because we can rely on its default
second parameter.

Author: kazutakahirata

clang/lib/Driver/Job.cpp

@@ -184,7 +184,7 @@ rewriteIncludes(const llvm::ArrayRef<const char *> &Args, size_t Idx,
     StringRef FlagRef(Args[Idx + NumArgs - 1]);
     assert((FlagRef.starts_with("-F") || FlagRef.starts_with("-I")) &&
            "Expecting -I or -F");
-    StringRef Inc = FlagRef.slice(2, StringRef::npos);
+    StringRef Inc = FlagRef.substr(2);
     if (getAbsPath(Inc, NewInc)) {
       SmallString<128> NewArg(FlagRef.slice(0, 2));
       NewArg += NewInc;

clang/lib/Driver/ToolChain.cpp

@@ -1441,7 +1441,7 @@ std::string ToolChain::detectLibcxxVersion(StringRef IncludePath) const {
     StringRef VersionText = llvm::sys::path::filename(LI->path());
     int Version;
     if (VersionText[0] == 'v' &&
-        !VersionText.slice(1, StringRef::npos).getAsInteger(10, Version)) {
+        !VersionText.substr(1).getAsInteger(10, Version)) {
       if (Version > MaxVersion) {
         MaxVersion = Version;
         MaxVersionString = std::string(VersionText);