[SIL] Don't print duplicated import decls

[kitasuke]

Issue

If we pass a raw SIL file as input to sil-opt, we get duplicated import declarations like below.
Here is commands to reproduce

sil-opt

$sil-opt input.sil > output.sil

input.sil

sil_stage raw

import Builtin
import Swift
import SwiftShims

func foo() {}
...

output.sil

sil_stage raw

import Builtin
import Swift
import SwiftShims

import Builtin

import Swift

import SwiftShims

func foo() {}
...

swiftc

$swiftc -emit-sil input.sil > output.sil

input.sil

sil_stage raw

import Builtin
import Swift
import SwiftShims

func foo() {}
...

output.sil

sil_stage canonical

import Builtin
import Swift
import SwiftShims

import Builtin

import Swift

import SwiftShims

func foo() {}
...

Why

swiftc goes through ASTPrinter for each decl including ImportDecl in SILPrinter. So if we already have import decls in silgen, it prints the import decls twice in SILPrinter and ASTPrinter.

Fix

Let's skip to visit visitImportDecl so that we wouldn't have the duplicated import decls.

[slavapestov]

Can you add a test?

[kitasuke]

I added test/sil-opt/import-decls.silgen, but was it not enough to check import declarations?

[kitasuke]

I found out that this is not a specific issue for sil-opt. Same output is emitted when swiftc takes raw SIL to emit canonical SIL. Is it more appropriate to put the test case in test/SIL then?

[slavapestov]

This test will still pass with your change reverted. You need to build and import a custom module. And as you said this is not specific to sil-opt and should be moved to test/SIL.

[kitasuke]

Addressed your feedback. Hope I correctly understood what you suggested.

[slavapestov]

Looks good, thanks!

[slavapestov]

@swift-ci Please smoke test

[kitasuke]

I really appreciate for your all the feedbacks!