[Serialization] Improve extensions of nested types with the same name (#7397)

Previously looking up an extension would result in all extensions for
types with the same name (nested or not) being deserialized; this
could even bring in base types that had not been deserialized yet. Add
in a string to distinguish an extension's base type; in the top-level
case this is just a module name, but for nested types it's a full
mangled name.

This is a little heavier than I'd like it to be, since it means we
mangle names and then throw them away, and since it means there's a
whole bunch of extra string data in the module just for uniquely
identifying a declaration. But it's correct, and does less work than
before, and fixes a circularity issue with a nested type A.B.A that
apparently used to work.

https://bugs.swift.org/browse/SR-3915

include/swift/Serialization/ModuleFormat.h

@@ -54,7 +54,7 @@ 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 = 314; // Last change: change synthetic substitution serialization
+const uint16_t VERSION_MINOR = 315; // Last change: uniquely identify extensions
 
 using DeclID = PointerEmbeddedInt<unsigned, 31>;
 using DeclIDField = BCFixed<31>;
@@ -1476,6 +1476,12 @@ namespace index_block {
     BCBlob       // actual names
   >;
 
+  using ExtensionTableLayout = BCRecordLayout<
+    EXTENSIONS, // record ID
+    BCVBR<16>,  // table offset within the blob (see below)
+    BCBlob  // map from identifier strings to decl kinds / decl IDs
+  >;
+
   using ObjCMethodTableLayout = BCRecordLayout<
     OBJC_METHODS,  // record ID
     BCVBR<16>,     // table offset within the blob (see below)