[Serialization] Record the new SIL_EXTRA_STRING block

Also fix up a test

Author: DougGregor

lib/Serialization/Serialization.cpp

@@ -975,7 +975,7 @@ void Serializer::writeBlockInfoBlock() {
   BLOCK_RECORD(sil_block, SIL_SOURCE_LOC_REF);
   BLOCK_RECORD(sil_block, SIL_DEBUG_VALUE);
   BLOCK_RECORD(sil_block, SIL_DEBUG_VALUE_DELIMITER);
-
+  BLOCK_RECORD(sil_block, SIL_EXTRA_STRING);
 
   BLOCK(SIL_INDEX_BLOCK);
   BLOCK_RECORD(sil_index_block, SIL_FUNC_NAMES);

test/embedded/wrong-linkage.swift

@@ -4,6 +4,6 @@
 // REQUIRES: OS=macosx
 
 @_cdecl("posix_memalign")
-func posix_memalign(_ resultPtr:UnsafeMutablePointer<UnsafeMutableRawPointer?>, _ :Int, _ :Int) -> Int32 { // expected-error {{function has wrong linkage to be called from}}
+func posix_memalign(_ resultPtr:UnsafeMutablePointer<UnsafeMutableRawPointer?>, _ :Int, _ :Int) -> Int32 { // okay: @_extern(c) declaration is separate
   return 0
 }