Sema: Remove DeclChecker::IsSecondPass

The DeclChecker had three possible states:

- IsFirstPass true, IsSecondPass false. This is the 'first pass' for
  declarations that appear at the top-level, or are nested inside
  top-level types.

- IsFirstPass false, IsSecondPass true. This is the 'second pass' for
  declarations that appear at the top-level, or are nested inside
  top-level types.

- IsFirstPass false, IsSecondPass false. This was used for (some)
  local declarations.

This is unnecessarily confusing. We can eliminate the third state
by calling typeCheckDecl() twice in a few places. This allows
IsSecondPass to be removed entirely since it's now always equal to
!IsFirstPass.

lib/Sema/CodeSynthesis.cpp

@@ -1975,7 +1975,7 @@ ConstructorDecl *swift::createImplicitConstructor(TypeChecker &tc,
   }
 
   // Type-check the constructor declaration.
-  tc.typeCheckDecl(ctor, /*isFirstPass=*/true);
+  tc.validateDecl(ctor);
 
   return ctor;
 }