Fix spacing between multiple @_spi(...) attributes.

This is a companion to swiftlang/swift-format#501,
which tweaks the breaking behavior for `import` decls (and also fixes
this existing mis-styling).

Author: allevato

Tests/SwiftParserTest/Assertions.swift

@@ -12,7 +12,7 @@
 
 import XCTest
 @_spi(RawSyntax) import SwiftSyntax
-@_spi(Testing)@_spi(RawSyntax) import SwiftParser
+@_spi(Testing) @_spi(RawSyntax) import SwiftParser
 @_spi(RawSyntax) import SwiftParserDiagnostics
 import SwiftDiagnostics
 import _SwiftSyntaxTestSupport

Tests/SwiftParserTest/DeclarationTests.swift

@@ -11,7 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 @_spi(RawSyntax) import SwiftSyntax
-@_spi(Testing)@_spi(RawSyntax) import SwiftParser
+@_spi(Testing) @_spi(RawSyntax) import SwiftParser
 import SwiftSyntaxBuilder
 import SwiftBasicFormat
 import XCTest