[32-bit Linux] Align _GenericContext to 8 bytes

A static assert fires around GenericContext not quite being the correct size for 32-bit. Aligning _GenericContext addresses this problem.

swift/include/swift/AST/Decl.h:
 1541  static_assert(sizeof(_GenericContext) + sizeof(DeclContext) ==
 1542:               sizeof(GenericContext), "Please add fields to _GenericContext");

Author: Kaiede

include/swift/AST/Decl.h

@@ -1462,7 +1462,7 @@ class alignas(RequirementRepr) TrailingWhereClause final :
 };
 
 // A private class for forcing exact field layout.
-class _GenericContext {
+class alignas(8) _GenericContext {
 // Not really public. See GenericContext.
 public:
   GenericParamList *GenericParams = nullptr;