Initial implementation of a @_cdecl attribute to export top-level functions to C.

There's an immediate need for this in the core libs, and we have most of the necessary pieces on hand to make it easy to implement. This is an unpolished initial implementation, with the following limitations, among others:

- It doesn't support bridging error conventions,
- It relies on ObjC interop,
- It doesn't check for symbol name collisions,
- It has an underscored name with required symbol name `@cdecl("symbol_name")`, awaiting official bikeshed painting.

lib/Serialization/Deserialization.cpp

@@ -1884,6 +1884,14 @@ Decl *ModuleFile::getDecl(DeclID DID, Optional<DeclContext *> ForcedContext) {
         break;
       }
 
+      case decls_block::CDecl_DECL_ATTR: {
+        bool isImplicit;
+        serialization::decls_block::CDeclDeclAttrLayout::readRecord(
+            scratch, isImplicit);
+        Attr = new (ctx) CDeclAttr(blobData, isImplicit);
+        break;
+      }
+
       case decls_block::Alignment_DECL_ATTR: {
         bool isImplicit;
         unsigned alignment;