[serialization] Stop recording the full paths to input source files.

There were no clients and it leaks information about the developer's system.
After this commit, there should be no full paths present in framework modules.
(App modules may contain search paths for debugging reasons, as well as a
full or relative path to the bridging header.)

Swift SVN r24851

include/swift/Serialization/ModuleFormat.h

@@ -51,7 +51,7 @@ const uint16_t VERSION_MAJOR = 0;
 /// To ensure that two separate changes don't silently get merged into one
 /// in source control, you should also update the comment to briefly
 /// describe what change you made.
-const uint16_t VERSION_MINOR = 169; // Last change: local type serialization
+const uint16_t VERSION_MINOR = 170; // Last change: dropped SOURCE_FILE record.
 
 using DeclID = Fixnum<31>;
 using DeclIDField = BCFixed<31>;
@@ -391,8 +391,7 @@ namespace input_block {
   // These IDs must \em not be renumbered or reordered without incrementing
   // VERSION_MAJOR.
   enum {
-    SOURCE_FILE = 1,
-    IMPORTED_MODULE,
+    IMPORTED_MODULE = 1,
     LINK_LIBRARY,
     IMPORTED_HEADER,
     IMPORTED_HEADER_CONTENTS,
@@ -400,11 +399,6 @@ namespace input_block {
     SEARCH_PATH
   };
 
-  using SourceFileLayout = BCRecordLayout<
-    SOURCE_FILE, // ID
-    BCBlob // path
-  >;
-
   using ImportedModuleLayout = BCRecordLayout<
     IMPORTED_MODULE,
     BCFixed<1>, // exported?