Add "interface hash" for improved incremental builds.

Compute the hash of all interface tokens when parsing; write the
interface hash to the swiftdeps file, or if the -dump-interface-hash
option is passed to the frontend. This hash will be used in incremental
mode to determine whether a file's interface has changed, and therefore
whether dependent files need to be rebuilt in response to the change.

Committed on ChrisW's behalf while he gets his setup unborked.

rdar://problem/15352929

Swift SVN r30477

include/swift/Frontend/FrontendOptions.h

@@ -126,6 +126,7 @@ public:
     NoneAction, ///< No specific action
     Parse, ///< Parse and type-check only
     DumpParse, ///< Parse only and dump AST
+    DumpInterfaceHash, ///< Parse and dump the interface token hash.
     DumpAST, ///< Parse, type-check, and dump AST
     PrintAST, ///< Parse, type-check, and pretty-print AST