Bridging: Implement bridges required for ongoing AutoDiff changes

In #83926, part of the changes resolving #68944 is submitted. The AutoDiff
closure specialization optimizer pass relies on several bridges not
implemented yet.

This patch introduces these missing bridges. See the list of the changes below.

**AST:**

* Define `.asValueDecl` on each BridgedXXXDecl type that's also a ValueDecl.

* Define `.asNominalTypeDecl` on each BridgedXXXDecl type that's also a NominalTypeDecl.

* Define `.asGenericContext` on each BridgedXXXDecl type that's also a GenericContext.

* `class BridgedSourceFile`:
  - Make nullable
  - `func addTopLevelDecl(_ decl: BridgedDecl)`

* `class BridgedFileUnit`:
  - `func castToSourceFile() -> BridgedNullableSourceFile`

* `class BridgedDecl`:
  - `func getDeclContext() -> BridgedDeclContext`

* `class BridgedParamDecl`:
  - `func cloneWithoutType() -> BridgedParamDecl`
  - `func setInterfaceType(_ type : BridgedASTType)`

* Define `BridgedDecl.setImplicit()` instead of `BridgedParamDecl.setImplicit()`

* `class BridgedGenericContext`:
  - `setGenericSignature(_ genSig: BridgedGenericSignature)`

* Change return type of `BridgedEnumDecl.createParsed(/*...*/)` from `BridgedNominalTypeDecl` to `BridgedEnumDecl`

* `class BridgedValueDecl`:
  - `func setAccess(_ accessLevel: swift.AccessLevel)`

* `class BridgedNominalTypeDecl`:
  - `func addMember(_ member: BridgedDecl)`

* `class BridgedGenericTypeParamDecl`:
  - `func createImplicit(declContext: BridgedDeclContext, name: swift.Identifier, depth: UInt, index: UInt, paramKind: swift.GenericTypeParamKind)`

* `class ValueDecl`:
  - `var baseIdentifier: swift.Identifier`

* `class NominalTypeDecl`:
  - `var declaredInterfaceType: Type`

* `class EnumElementDecl`:
  - `var parameterList: BridgedParameterList`
  - `var nameStr: StringRef`

* `struct GenericSignature`:
  - `var canonicalSignature: CanGenericSignature`

* `struct CanGenericSignature`:
  - `var isEmpty: Bool`
  - `var genericSignature: GenericSignature`

* `struct Type`:
  - `func mapTypeOutOfContext() -> Type`
  - `func getReducedType(sig: GenericSignature) -> CanonicalType`
  - `func GenericTypeParam_getName() -> swift.Identifier`
  - `func GenericTypeParam_getDepth() -> UInt`
  - `func GenericTypeParam_getIndex() -> UInt`
  - `func GenericTypeParam_getParamKind() -> swift.GenericTypeParamKind`

* `struct CanonicalType`:
  - `func SILFunctionType_getSubstGenericSignature() -> CanGenericSignature`
  - `func loweredType(in function: SIL.Function) -> SIL.Type`

**SIL:**

* `class Argument`:
  - `func replaceAllUsesWith(newArg: Argument)`

* `class BasicBlock`:
  - `func insertPhiArgument(atPosition: Int, type: Type, ownership: Ownership, _ context: some MutatingContext) -> Argument`

* `struct Builder`:
  - `func createTuple(elements: [Value]) -> TupleInst`

* `protocol Context`:
  - `func getTupleType(elements: [AST.Type]) -> AST.Type`
  - `func getTupleTypeWithLabels(elements: [AST.Type], labels: [swift.Identifier]) -> AST.Type`

* `class Function`:
  - `var sourceFile: BridgedNullableSourceFile`
  - `func mapTypeIntoContext(_ type: Type) -> Type`

* `class PartialApplyInst`:
  - `var substitutionMap: SubstitutionMap`

* `class SwitchEnumInst`:
  - `var numCases: Int`
  - `public func getSuccessorForDefault() -> BasicBlock?`

* `Type`:
  - `var category: ValueCategory`
  - `func getEnumCasePayload(caseIdx: Int, function: Function) -> Type`
  - `func mapTypeOutOfContext() -> Type`
  - `static func getPrimitiveType(canType: CanonicalType, silValueCategory: ValueCategory) -> Type`

* `struct EnumCase`:
  - `let enumElementDecl: EnumElementDecl`

* `struct TupleElementArray`:
  - `func label(at index: Int) -> swift.Identifier`

* Define `enum ValueCategory` with `address` and `object` elements

SwiftCompilerSources/Sources/SIL/BasicBlock.swift

@@ -68,6 +68,11 @@ final public class BasicBlock : CustomStringConvertible, HasShortDescription, Ha
                                           (decl as Decl?).bridged).argument as! FunctionArgument
   }
 
+  public func insertPhiArgument(atPosition: Int, type: Type, ownership: Ownership, _ context: some MutatingContext) -> Argument {
+    context.notifyInstructionsChanged()
+    return bridged.insertPhiArgument(atPosition, type.bridged, ownership._bridged).argument
+  }
+
   public func eraseArgument(at index: Int, _ context: some MutatingContext) {
     context.notifyInstructionsChanged()
     bridged.eraseArgument(index)