Update the PlaceholderFinder ASTWalker to walk into the clauses of
IfConfigDecls. It wasn't previously, resulting in any placeholders there not
being expanded.
Also update CallExprFinder (used to determine if expansions should use trailing
closure syntax) to walk into inactive if-config clauses. Previously it only
walked into active regions, so expansions never used trailing closure syntax in
inactive regions.
Resolves rdar://problem/51995648