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

include/swift/AST/SILOptions.h

@@ -62,6 +62,9 @@ public:
   /// Useful when you want to enable -O LLVM opts but not -O SIL opts.
   bool DisableSILPerfOptimizations = false;
 
+  /// Controls whether cross module optimization is enabled.
+  bool CrossModuleOptimization = false;
+  
   /// Controls whether or not paranoid verification checks are run.
   bool VerifyAll = false;
 

include/swift/Option/Options.td

@@ -589,6 +589,10 @@ def Oplayground : Flag<["-"], "Oplayground">, Group<O_Group>,
   Flags<[HelpHidden, FrontendOption, ModuleInterfaceOption]>,
   HelpText<"Compile with optimizations appropriate for a playground">;
 
+def CrossModuleOptimization : Flag<["-"], "cross-module-optimization">,
+  Flags<[HelpHidden, FrontendOption]>,
+  HelpText<"Perform cross-module optimization">;
+
 def RemoveRuntimeAsserts : Flag<["-"], "remove-runtime-asserts">,
   Flags<[FrontendOption]>,
   HelpText<"Remove runtime safety checks.">;

include/swift/SILOptimizer/PassManager/Passes.def

@@ -64,6 +64,8 @@ PASS(AccessEnforcementSelection, "access-enforcement-selection",
      "Access Enforcement Selection")
 PASS(AccessEnforcementWMO, "access-enforcement-wmo",
      "Access Enforcement Whole Module Optimization")
+PASS(CrossModuleSerializationSetup, "cross-module-serialization-setup",
+     "Setup serialization flags for cross-module optimization")
 PASS(AccessSummaryDumper, "access-summary-dump",
      "Dump Address Parameter Access Summary")
 PASS(AccessedStorageDumper, "accessed-storage-dump",