Avoid using system isdigit in favor of swift::Mangle::isDigit (swiftlang#39382)

Author: kubamracek

lib/Demangling/Context.cpp

@@ -16,6 +16,7 @@
 
 #include "swift/Demangling/Demangler.h"
 #include "swift/Demangling/ManglingMacros.h"
+#include "swift/Demangling/ManglingUtils.h"
 #include "swift/Demangling/NamespaceMacros.h"
 
 namespace swift {
@@ -76,7 +77,7 @@ std::string Context::demangleTypeAsString(llvm::StringRef MangledName,
 static llvm::StringRef stripSuffix(llvm::StringRef Name) {
   // A suffix always ends with a digit. Do this quick check to avoid scanning through the whole
   // symbol name if the symbol has no suffix (= the common case).
-  if (isdigit(Name.back())) {
+  if (swift::Mangle::isDigit(Name.back())) {
     size_t dotPos = Name.find('.');
     if (dotPos != StringRef::npos) {
       Name = Name.substr(0, dotPos);

lib/Demangling/OldRemangler.cpp

@@ -1103,7 +1103,8 @@ void Remangler::mangleNamedEntity(Node *node, char basicKind,
   Buffer << entityKind;
 
   auto privateDiscriminator = ctx.takeAnonymousContextDiscriminator();
-  if (!privateDiscriminator.empty() && isdigit(privateDiscriminator[0]))
+  if (!privateDiscriminator.empty() &&
+      swift::Mangle::isDigit(privateDiscriminator[0]))
     privateDiscriminator = "_" + privateDiscriminator;
   if (!artificialPrivateDiscriminator.empty())
     privateDiscriminator.append(artificialPrivateDiscriminator.data(),