[flang] Remove redundant string initialization (NFC)

Identified with readability-redundant-string-init.

flang/examples/FlangOmpReport/FlangOmpReportVisitor.h

@@ -80,7 +80,7 @@ struct OpenMPCounterVisitor {
   void Post(const OmpClause &c);
   void PostClauseCommon(const ClauseInfo &ci);
 
-  std::string clauseDetails{""};
+  std::string clauseDetails;
   llvm::SmallVector<LogRecord> constructClauses;
   llvm::SmallVector<OmpWrapperType *> ompWrapperStack;
   llvm::DenseMap<OmpWrapperType *, llvm::SmallVector<ClauseInfo>> clauseStrings;

flang/lib/Lower/Mangler.cpp

@@ -222,7 +222,7 @@ std::string Fortran::lower::mangle::mangleArrayLiteral(
     const Fortran::evaluate::ConstantSubscripts &shape,
     Fortran::common::TypeCategory cat, int kind,
     Fortran::common::ConstantSubscript charLen) {
-  std::string typeId = "";
+  std::string typeId;
   for (Fortran::evaluate::ConstantSubscript extent : shape)
     typeId.append(std::to_string(extent)).append("x");
   if (charLen >= 0)

flang/lib/Optimizer/Support/InternalNames.cpp

@@ -329,7 +329,7 @@ static std::string
 mangleTypeDescriptorKinds(llvm::ArrayRef<std::int64_t> kinds) {
   if (kinds.empty())
     return "";
-  std::string result = "";
+  std::string result;
   for (std::int64_t kind : kinds)
     result += "." + std::to_string(kind);
   return result;

flang/lib/Semantics/tools.cpp

@@ -1294,7 +1294,7 @@ template <ComponentKind componentKind>
 std::string
 ComponentIterator<componentKind>::const_iterator::BuildResultDesignatorName()
     const {
-  std::string designator{""};
+  std::string designator;
   for (const auto &node : componentPath_) {
     designator += "%" + DEREF(node.component()).name().ToString();
   }