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.

include/swift/Serialization/ModuleFormat.h

@@ -43,7 +43,7 @@ const unsigned char MODULE_DOC_SIGNATURE[] = { 0xE2, 0x9C, 0xA8, 0x07 };
 
 /// Serialized module format major version number.
 ///
-/// Always 0 for Swift 1.0.
+/// Always 0 for Swift 1.x and 2.x.
 const uint16_t VERSION_MAJOR = 0;
 
 /// Serialized module format minor version number.
@@ -53,7 +53,8 @@ const uint16_t VERSION_MAJOR = 0;
 /// in source control, you should also update the comment to briefly
 /// describe what change you made. The content of this comment isn't important;
 /// it just ensures a conflict if two people change the module format.
-const uint16_t VERSION_MINOR = 239; // multiple results for SILFunctionType
+/// describe what change you made.
+const uint16_t VERSION_MINOR = 240; // Last change: @_cdecl
 
 using DeclID = PointerEmbeddedInt<unsigned, 31>;
 using DeclIDField = BCFixed<31>;
@@ -1223,6 +1224,13 @@ namespace decls_block {
     BCFixed<1>, // implicit flag
     BCBlob      // _silgen_name
   >;
+
+  using CDeclDeclAttrLayout = BCRecordLayout<
+    CDecl_DECL_ATTR,
+    BCFixed<1>, // implicit flag
+    BCBlob      // _silgen_name
+  >;
+
   
   using AlignmentDeclAttrLayout = BCRecordLayout<
     Alignment_DECL_ATTR,