[LLD] [COFF] Add a couple "MinGW only" comments re linking against DLLs. NFC.

This was requested in the post-commit review of D104530.

lld/COFF/InputFiles.h

@@ -395,7 +395,7 @@ private:
   std::vector<Symbol *> symbols;
 };
 
-// .dll file.
+// .dll file. MinGW only.
 class DLLFile : public InputFile {
 public:
   explicit DLLFile(MemoryBufferRef m) : InputFile(DLLKind, m) {}

lld/COFF/Symbols.h

@@ -311,6 +311,7 @@ public:
   LazyObjFile *file;
 };
 
+// MinGW only.
 class LazyDLLSymbol : public Symbol {
 public:
   LazyDLLSymbol(DLLFile *f, DLLFile::Symbol *s, StringRef n)