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

lib/Frontend/FrontendOptions.cpp

@@ -22,6 +22,7 @@ bool FrontendOptions::actionHasOutput() const {
   case Parse:
   case DumpParse:
   case DumpAST:
+  case DumpInterfaceHash:
   case PrintAST:
   case DumpTypeRefinementContexts:
     return false;
@@ -49,6 +50,7 @@ bool FrontendOptions::actionIsImmediate() const {
   case Parse:
   case DumpParse:
   case DumpAST:
+  case DumpInterfaceHash:
   case PrintAST:
   case DumpTypeRefinementContexts:
   case EmitSILGen: