[NFC] Use DeclAttr.def flags for @abi checking

Information about this behavior is needed in other parts of the compiler, so let’s put it somewhere where we can get at it.

The only test changes in this commit are to move comments out of the decl checker and into the test, since there is no longer anywhere else to put them.

Author: beccadax

include/swift/AST/Attr.h

@@ -364,6 +364,41 @@ class DeclAttribute : public AttributeBase {
 
     /// The opposite of ABIBreakingToRemove
     ABIStableToRemove = 1ull << 15,
+
+    /// Attribute should not be used in an \c \@abi attribute. Use for
+    /// attributes which cannot affect mangled names, even indirectly, and
+    /// which either don't affect ABI or where ABI-only declarations get their
+    /// behavior from their API counterpart.
+    ForbiddenInABIAttr = 1ull << 16,
+
+    /// Attribute can be used without restrictions in an \c \@abi attribute.
+    /// Use for attributes which affect mangled names but otherwise don't alter
+    /// the ABI, or ones where the \c ABIDeclChecker manually implements
+    /// special checking logic (e.g. because several different attributes
+    /// contribute to the same aspect of ABI in some complicated way).
+    UnconstrainedInABIAttr = 1ull << 17,
+
+    /// Attribute can be used in an \c \@abi attribute, but must match
+    /// equivalent on API decl. Use for attributes which affect both mangled
+    /// names and other parts of the ABI such that the declaration can only be
+    /// valid if they match. 
+    EquivalentInABIAttr = 1ull << 18,
+
+    /// Attribute can be used in an \c \@abi attribute, but must match
+    /// equivalent on API decl; if omitted, API decl's attribute will be 
+    /// cloned. Use where you would want to use \c EquivalentInABIAttr but 
+    /// repeating the attribute is judged too burdensome.
+    InferredInABIAttr = 1ull << 19,
+
+    /// Use for attributes which are \em only valid on declarations that cannot
+    /// have an \c @abi attribute, such as \c ImportDecl .
+    UnreachableInABIAttr = 1ull << 20,
+  };
+
+  enum : uint64_t {
+    InABIAttrMask = ForbiddenInABIAttr | UnconstrainedInABIAttr
+                  | EquivalentInABIAttr | InferredInABIAttr
+                  | UnreachableInABIAttr
   };
 
   LLVM_READNONE