[AutoDiff] Fix "'alignof' to an incomplete type" error in AutoDiff.h.

[dan-zheng]

Using DenseMap<SILAutoDiffDerivativeFunctionKey, ...> in a file that includes
AutoDiff.h but not Types.h produced the following error:

swift-master/llvm-project/llvm/include/llvm/Support/PointerLikeTypeTraits.h:59:53: error: invalid application of 'alignof' to an incomplete type 'swift::SILFunctionType'
  enum { NumLowBitsAvailable = detail::ConstantLog2<alignof(T)>::value };
                                                    ^~~~~~~~~~
swift-master/llvm-project/llvm/include/llvm/ADT/DenseMapInfo.h:43:13: note: in instantiation of template class 'llvm::PointerLikeTypeTraits<swift::SILFunctionType *>' requested here
    Val <<= PointerLikeTypeTraits<T*>::NumLowBitsAvailable;
            ^
swift-master/swift/include/swift/AST/AutoDiff.h:383:46: note: in instantiation of member function 'llvm::DenseMapInfo<swift::SILFunctionType *>::getEmptyKey' requested here
    return {DenseMapInfo<SILFunctionType *>::getEmptyKey(),
                                             ^
swift-master/swift/include/swift/AST/AutoDiff.h:33:7: note: forward declaration of 'swift::SILFunctionType'
class SILFunctionType;
      ^

Fix the error by:

• Including TypeAlignments.h in AutoDiff.h.
• Adding an alignment forward declaration for SILFunctionType to TypeAlignments.h.

---

Resolves comment: https://github.com/apple/swift/pull/29953/files#r382372071.
No tests, since the error didn't surface in the current codebase.

I locally verified that the issue is fixed. The following changes didn't compile but now do:

diff --git a/lib/AST/AutoDiffTest.cpp b/lib/AST/AutoDiffTest.cpp
new file mode 100644
index 00000000000..ab1d4caa278
--- /dev/null
+++ b/lib/AST/AutoDiffTest.cpp
@@ -0,0 +1,9 @@
+#include "swift/AST/AutoDiff.h"
+#include "llvm/ADT/DenseMap.h"
+
+using namespace swift;
+using namespace autodiff;
+
+static void test() {
+  llvm::DenseMap<SILAutoDiffDerivativeFunctionKey, unsigned> map;
+}
diff --git a/lib/AST/CMakeLists.txt b/lib/AST/CMakeLists.txt
index 0add9c6606c..5c867797a28 100644
--- a/lib/AST/CMakeLists.txt
+++ b/lib/AST/CMakeLists.txt
@@ -27,6 +27,7 @@ add_swift_host_library(swiftAST STATIC
   ASTWalker.cpp
   Attr.cpp
   AutoDiff.cpp
+  AutoDiffTest.cpp
   Availability.cpp
   AvailabilitySpec.cpp
   Builtins.cpp

[milseman]

@swift-ci please smoke test

[milseman]

Thank you!