Demangling: remove stray llvm forward declaration

Remove the forward declaration for `llvm::raw_ostream` as this creates
ambiguity for `__swift::__runtime::llvm::StringRef` and
`llvm::StringRef` as now `llvm` is made visible to the namespace lookup
rules.

Author: compnerd

include/swift/Demangling/Demangle.h

@@ -30,10 +30,6 @@
 #include <memory>
 #include <string>
 
-namespace llvm {
-  class raw_ostream;
-}
-
 namespace swift {
 namespace Demangle {
 SWIFT_BEGIN_INLINE_NAMESPACE