[IDE] SourceEntityWalker should walk all explicit declarations

`SourceEntityWalker` had an unbalanced `walkToDeclPre` and
`walkToDeclPost`, ie. `walkToDeclPost` could be called even though
`walkToDeclPre` was not. Specifically, this would occur for both
`OperatorDecl` and `PrecedenceGroupDecl` declarations.

These could both be added to the `if` in `walkToDeclPost`, but this
seems fairly errorprone in general - especially as new decls are added.
Indeed, there's already declarations that are being skipped because they
aren't explicitly tested for in `walkToDeclPre`, ie.
`PatternBindingDecl`.

Instead of skipping if not explcitly handled, only skip running the
`SEWalker` walk methods if the declaration is implicit (and not a
constructor decl, see TODO). This should probably also always visit
children, with various decls changed to become implicit (eg.
TopLevelCodeDecl), but we can do that later - breaks too many tests for
now.

This change exposed a few parameter declarations that were missing their
implicit flag, as well as unbalanced walk methods in `RangeResolver`.

lib/Sema/DerivedConformanceAdditiveArithmetic.cpp

@@ -181,6 +181,7 @@ static ValueDecl *deriveMathOperator(DerivedConformance &derived,
                           C.getIdentifier(name), parentDC);
     param->setSpecifier(ParamDecl::Specifier::Default);
     param->setInterfaceType(type);
+    param->setImplicit();
     return param;
   };