[AST] Fix setting of DWARFMangling in ASTMangler

Turns out this constructor was never actually setting this bit.
`mangleTypeForDebugger` also sets this to `true`, so it doesn't seem
like this caused issues in practice, but let's actually set it. This
parameter seems like it could be removed at this point, but I'll
leave that for a follow-up.

Author: hamishknight

include/swift/AST/ASTMangler.h

@@ -187,10 +187,10 @@ class ASTMangler : public Mangler {
     HasSymbolQuery,
   };
 
-  /// lldb overrides the defaulted argument to 'true'.
+  /// lldb overrides \p DWARFMangling to 'true'.
   ASTMangler(const ASTContext &Ctx, bool DWARFMangling = false) : Context(Ctx) {
     if (DWARFMangling) {
-      DWARFMangling = true;
+      this->DWARFMangling = true;
       RespectOriginallyDefinedIn = false;
     }
     Flavor = Ctx.LangOpts.hasFeature(Feature::Embedded)