[Driver] add InputFileKind to FrontendOptions.

Separate InputFileKind from SourceFileKind, FrontendOptions will now use
InputFileKind, while Module will use SourceFileKind.

This is in preparation for adding an input file kind for LLVM IR.

rdar://19048891


Swift SVN r25555

tools/swift-ide-test/ModuleAPIDiff.cpp

@@ -951,7 +951,8 @@ int swift::doGenerateModuleAPIDescription(StringRef MainExecutablePath,
   PrintOptions Options = PrintOptions::printEverything();
 
   Module *M = CI.getMainModule();
-  M->getMainSourceFile(Invocation->getInputKind()).print(llvm::outs(), Options);
+  M->getMainSourceFile(Invocation->getSourceFileKind()).print(llvm::outs(),
+                                                        Options);
 
   auto SMAModel = createSMAModel(M);
   llvm::yaml::Output YOut(llvm::outs());