[5.1][SourceKit] Fix placeholder expansion not working inside #if

[nathawes]

Cherry-pick of #26036 from master

• Issue: rdar://problem/51995648

• Explanation:
  All editor placeholder expansions inside #if blocks were failing. For example, with the below placeholder produced by code completion:

  #if DEBUG
  func test2() {
      [1,2,3].filter(<#T##isIncluded: (Int) throws -> Bool##(Int) throws -> Bool#>)
  }
  #endif
  

  the expansion was failing and Xcode would simply insert the placeholder content verbatim:

  [1,2,3].filter(#T##isIncluded: (Int) throws -> Bool##(Int) throws -> Bool#)
  

  rather than expanding successfully as it does outside of #if blocks:

  [1,2,3].filter { (<#Int#>) -> Bool in
      <#code#>
  }
  

  This was failing because the PlaceholderFinder ASTWalker wasn't walking into the clauses of IfConfigDecls, so didn't find them. Additionally the CallExprFinder walker (used to determine if expansions should use trailing closure syntax) wasn't configured to walk into inactive if-config clauses, so expansions weren't using trailing closure syntax in inactive regions.

• Scope: This issue affects all editor placeholder expansions inside of #if blocks

• Risk: Low. The fix is limited to SourceKit's placeholder expansion request, and only affects the behavior on placeholders within #if blocks.

• Origination: Placeholder expansion never worked in inactive #if blocks, but used to work in active #if blocks prior to Xcode 10.2. The offending change was that #if conditions are no longer evaluated when parsing to produce an AST for SourceKit's purely syntactic requests (including placeholder expansion) so there aren't any active clauses anymore.

• Testing: Added regression tests and manually checked it behaves correctly in Xcode

• Reviewer: Xi Ge (in the master PR)

Resolves rdar://problem/51995648

[nathawes]

@swift-ci please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - f723c8e

[nathawes]

@swift-ci please test Linux

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - f723c8e

[nathawes]

@swift-ci please test Linux

[nathawes]

@swift-ci please nominate