Cross module optimization

This is a first version of cross module optimization (CMO).

The basic idea for CMO is to use the existing library evolution compiler features, but in an automated way. A new SIL module pass "annotates" functions and types with @inlinable and @usableFromInline. This results in functions being serialized into the swiftmodule file and thus available for optimizations in client modules.
The annotation is done with a worklist-algorithm, starting from public functions and continuing with entities which are used from already selected functions. A heuristic performs a preselection on which functions to consider - currently just generic functions are selected.

The serializer then writes annotated functions (including function bodies) into the swiftmodule file of the compiled module. Client modules are able to de-serialize such functions from their imported modules and use them for optimiations, like generic specialization.

The optimization is gated by a new compiler option -cross-module-optimization (also available in the swift driver).
By default this option is off. Without turning the option on, this change is (almost) a NFC.

rdar://problem/22591518

lib/FrontendTool/FrontendTool.cpp

@@ -1394,6 +1394,9 @@ static bool validateTBDIfNeeded(CompilerInvocation &Invocation,
   if (!astGuaranteedToCorrespondToSIL ||
       !inputFileKindCanHaveTBDValidated(Invocation.getInputKind()))
     return false;
+    
+  if (Invocation.getSILOptions().CrossModuleOptimization)
+    return false;
 
   const auto &frontendOpts = Invocation.getFrontendOptions();
   auto mode = frontendOpts.ValidateTBDAgainstIR;