[AST] suppress compiler warning

[aeu]

Issue

Warning when building the compiler

[1429/1722][ 82%][5851.879s] Building swift module AST
/mnt/storage/work/swift-project/swift/SwiftCompilerSources/Sources/AST/Declarations.swift:29:19: warning: treating a forced downcast to 'any DeclContext' as optional will nev
er produce 'nil'
 27 |   final public var parentDeclContext: DeclContext? {
 28 |     if let decl = bridged.getParent().decl {
 29 |       return decl as! DeclContext
    |              |    | `- note: use 'as?' to perform a conditional downcast to 'any DeclContext'
    |              |    `- warning: treating a forced downcast to 'any DeclContext' as optional will never produce 'nil'
    |              `- note: add parentheses around the cast to silence this warning
 30 |     }
 31 |     if let bridgedDeclContext = BridgedDeclContext(bridged: bridged.getDeclContext()) {

The compiler emits a diagnostic in parentDeclContext indicating that a forced cast is being treated as if it could produce nil. This occurs because the return type is DeclContext? and the ungrouped as! expression causes the warning.

Changes

Added parens to suppress the warning as recommended by the diagnostic.