[NFC] Change const char* to StringRef

This API takes a const char* with a default nullptr value and immdiately
passes it down to an API taking a StringRef. All of the places this is
called from are either using compile time string literals, the default
argument, or string objects that have known length. Discarding the
length known from a calling API to just have to strlen it to call the
next layer down that requires a StringRef is a bit silly, so this change
updates CodeGenModule::GetAddrOfConstantCString to use StringRef instead
of const char* for the GlobalName parameter.

It might be worth also replacing the first parameter with an llvm ADT
type that avoids allocation, but that change would have wider impact so
we should consider it separately.

Author: llvm-beanz

clang/lib/CodeGen/CGCUDANV.cpp

@@ -94,7 +94,7 @@ class CGNVCUDARuntime : public CGCUDARuntime {
   /// where the C code specifies const char*.
   llvm::Constant *makeConstantString(const std::string &Str,
                                      const std::string &Name = "") {
-    return CGM.GetAddrOfConstantCString(Str, Name.c_str()).getPointer();
+    return CGM.GetAddrOfConstantCString(Str, Name).getPointer();
   }
 
   /// Helper function which generates an initialized constant array from Str,

clang/lib/CodeGen/CGExpr.cpp

@@ -3513,11 +3513,11 @@ LValue CodeGenFunction::EmitPredefinedLValue(const PredefinedExpr *E) {
           CGM.getCXXABI().getMangleContext().getBlockId(BD, true);
       if (Discriminator)
         Name += "_" + Twine(Discriminator + 1).str();
-      auto C = CGM.GetAddrOfConstantCString(Name, GVName.c_str());
+      auto C = CGM.GetAddrOfConstantCString(Name, GVName);
       return MakeAddrLValue(C, E->getType(), AlignmentSource::Decl);
     } else {
       auto C =
-          CGM.GetAddrOfConstantCString(std::string(FnName), GVName.c_str());
+          CGM.GetAddrOfConstantCString(std::string(FnName), GVName);
       return MakeAddrLValue(C, E->getType(), AlignmentSource::Decl);
     }
   }

clang/lib/CodeGen/CGObjCGNU.cpp

@@ -195,7 +195,7 @@ class CGObjCGNU : public CGObjCRuntime {
   /// Helper function that generates a constant string and returns a pointer to
   /// the start of the string.  The result of this function can be used anywhere
   /// where the C code specifies const char*.
-  llvm::Constant *MakeConstantString(StringRef Str, const char *Name = "") {
+  llvm::Constant *MakeConstantString(StringRef Str, StringRef Name = "") {
     ConstantAddress Array =
         CGM.GetAddrOfConstantCString(std::string(Str), Name);
     return Array.getPointer();

clang/lib/CodeGen/CodeGenModule.cpp

@@ -6927,7 +6927,7 @@ CodeGenModule::GetAddrOfConstantStringFromObjCEncode(const ObjCEncodeExpr *E) {
 /// the literal and a terminating '\0' character.
 /// The result has pointer to array type.
 ConstantAddress CodeGenModule::GetAddrOfConstantCString(
-    const std::string &Str, const char *GlobalName) {
+    const std::string &Str, StringRef GlobalName) {
   StringRef StrWithNull(Str.c_str(), Str.size() + 1);
   CharUnits Alignment = getContext().getAlignOfGlobalVarInChars(
       getContext().CharTy, /*VD=*/nullptr);
@@ -6947,9 +6947,6 @@ ConstantAddress CodeGenModule::GetAddrOfConstantCString(
     }
   }
 
-  // Get the default prefix if a name wasn't specified.
-  if (!GlobalName)
-    GlobalName = ".str";
   // Create a global variable for this.
   auto GV = GenerateStringLiteral(C, llvm::GlobalValue::PrivateLinkage, *this,
                                   GlobalName, Alignment);

clang/lib/CodeGen/CodeGenModule.h

@@ -1188,7 +1188,7 @@ class CodeGenModule : public CodeGenTypeCache {
   /// created).
   ConstantAddress
   GetAddrOfConstantCString(const std::string &Str,
-                           const char *GlobalName = nullptr);
+                           StringRef GlobalName = ".str");
 
   /// Returns a pointer to a constant global variable for the given file-scope
   /// compound literal expression.