[AutoDiff] Fix "'alignof' to an incomplete type" error in AutoDiff.h. (#29983)

Using `DenseMap<SILAutoDiffDerivativeFunctionKey, ...>` in a file that includes
AutoDiff.h but not Types.h produced the following error:
```
error: invalid application of 'alignof' to an incomplete type 'swift::SILFunctionType'
```

Fix the error by:
- Including TypeAlignments.h in AutoDiff.h.
- Adding an alignment forward declaration for `SILFunctionType` to TypeAlignments.h.

Author: dan-zheng

include/swift/AST/AutoDiff.h

@@ -23,6 +23,7 @@
 #include "swift/AST/Identifier.h"
 #include "swift/AST/IndexSubset.h"
 #include "swift/AST/Type.h"
+#include "swift/AST/TypeAlignments.h"
 #include "swift/Basic/Range.h"
 #include "swift/Basic/SourceLoc.h"
 

include/swift/AST/TypeAlignments.h

@@ -46,6 +46,7 @@ namespace swift {
   class ProtocolDecl;
   class ProtocolConformance;
   class SILFunction;
+  class SILFunctionType;
   class Stmt;
   class TypeVariableType;
   class TypeBase;
@@ -100,6 +101,8 @@ LLVM_DECLARE_TYPE_ALIGNMENT(swift::ExtensionDecl, swift::DeclAlignInBits)
 LLVM_DECLARE_TYPE_ALIGNMENT(swift::TypeBase, swift::TypeAlignInBits)
 LLVM_DECLARE_TYPE_ALIGNMENT(swift::ArchetypeType, swift::TypeAlignInBits)
 LLVM_DECLARE_TYPE_ALIGNMENT(swift::TypeVariableType, swift::TypeVariableAlignInBits)
+LLVM_DECLARE_TYPE_ALIGNMENT(swift::SILFunctionType,
+                            swift::TypeVariableAlignInBits)
 
 LLVM_DECLARE_TYPE_ALIGNMENT(swift::Stmt, swift::StmtAlignInBits)
 LLVM_DECLARE_TYPE_ALIGNMENT(swift::BraceStmt, swift::StmtAlignInBits)