[clang] Drop explicit conversions of string literals to StringRef (NFC)

We routinely rely on implicit conversions of string literals to
StringRef so that we can use operator==(StringRef, StringRef).

The LHS here are all known to be of StringRef.

Author: kazutakahirata

clang/lib/Analysis/ThreadSafetyCommon.cpp

@@ -177,7 +177,7 @@ CapabilityExpr SExprBuilder::translateAttrExpr(const Expr *AttrExp,
     return CapabilityExpr();
 
   if (const auto* SLit = dyn_cast<StringLiteral>(AttrExp)) {
-    if (SLit->getString() == StringRef("*"))
+    if (SLit->getString() == "*")
       // The "*" expr is a universal lock, which essentially turns off
       // checks until it is removed from the lockset.
       return CapabilityExpr(new (Arena) til::Wildcard(), StringRef("wildcard"),

clang/lib/Sema/SemaDeclAttr.cpp

@@ -640,7 +640,7 @@ static void checkAttrArgsAreCapabilityObjs(Sema &S, Decl *D,
 
     if (const auto *StrLit = dyn_cast<StringLiteral>(ArgExp)) {
       if (StrLit->getLength() == 0 ||
-          (StrLit->isOrdinary() && StrLit->getString() == StringRef("*"))) {
+          (StrLit->isOrdinary() && StrLit->getString() == "*")) {
         // Pass empty strings to the analyzer without warnings.
         // Treat "*" as the universal lock.
         Args.push_back(ArgExp);

clang/lib/Serialization/ASTWriter.cpp

@@ -1460,7 +1460,7 @@ void ASTWriter::WriteControlBlock(Preprocessor &PP, ASTContext &Context,
         (!PP.getHeaderSearchInfo()
               .getHeaderSearchOpts()
               .ModuleMapFileHomeIsCwd ||
-         WritingModule->Directory->getName() != StringRef("."))) {
+         WritingModule->Directory->getName() != ".")) {
       // Module directory.
       auto Abbrev = std::make_shared<BitCodeAbbrev>();
       Abbrev->Add(BitCodeAbbrevOp(MODULE_DIRECTORY));