[flang] Resolve "possible performance problem" issue spam (#79769)

Four "issues" on GitHub report possible performance problems, likely
detected by static analysis. None of them would ever make a measureable
difference in compilation time, but I'm resolving them to clean up the
open issues list.

Fixes #79703, .../79705,
.../79706, & .../79707.

Author: klausler

flang/lib/Lower/OpenACC.cpp

@@ -3247,7 +3247,7 @@ static void createDeclareGlobalOp(mlir::OpBuilder &modBuilder,
                                   fir::FirOpBuilder &builder,
                                   mlir::Location loc, fir::GlobalOp globalOp,
                                   mlir::acc::DataClause clause,
-                                  const std::string declareGlobalName,
+                                  const std::string &declareGlobalName,
                                   bool implicit, std::stringstream &asFortran) {
   GlobalOp declareGlobalOp =
       modBuilder.create<GlobalOp>(loc, declareGlobalName);

flang/lib/Optimizer/CodeGen/Target.cpp

@@ -47,7 +47,7 @@ static const llvm::fltSemantics &floatToSemantics(const KindMapping &kindMap,
 }
 
 static void typeTodo(const llvm::fltSemantics *sem, mlir::Location loc,
-                     std::string context) {
+                     const std::string &context) {
   if (sem == &llvm::APFloat::IEEEhalf()) {
     TODO(loc, "COMPLEX(KIND=2): for " + context + " type");
   } else if (sem == &llvm::APFloat::BFloat()) {

flang/lib/Parser/preprocessor.cpp

@@ -252,7 +252,7 @@ void Preprocessor::DefineStandardMacros() {
   Define("__LINE__"s, "__LINE__"s);
 }
 
-void Preprocessor::Define(std::string macro, std::string value) {
+void Preprocessor::Define(const std::string &macro, const std::string &value) {
   definitions_.emplace(SaveTokenAsName(macro), Definition{value, allSources_});
 }
 

flang/lib/Parser/preprocessor.h

@@ -70,7 +70,7 @@ class Preprocessor {
   AllSources &allSources() { return allSources_; }
 
   void DefineStandardMacros();
-  void Define(std::string macro, std::string value);
+  void Define(const std::string &macro, const std::string &value);
   void Undefine(std::string macro);
   bool IsNameDefined(const CharBlock &);
   bool IsFunctionLikeDefinition(const CharBlock &);

flang/lib/Semantics/check-directive-structure.h

@@ -176,8 +176,8 @@ template <typename D, typename C, typename PC, std::size_t ClauseEnumSize>
 class DirectiveStructureChecker : public virtual BaseChecker {
 protected:
   DirectiveStructureChecker(SemanticsContext &context,
-      std::unordered_map<D, DirectiveClauses<C, ClauseEnumSize>>
-          directiveClausesMap)
+      const std::unordered_map<D, DirectiveClauses<C, ClauseEnumSize>>
+          &directiveClausesMap)
       : context_{context}, directiveClausesMap_(directiveClausesMap) {}
   virtual ~DirectiveStructureChecker() {}