libswift: Infrastructure to call libswift function passes from the SILOptimizer's PassManager

With the macro SWIFT_FUNCTION_PASS a new libswift function pass can be defined in Passes.def.
The SWIFT_FUNCTION_PASS_WITH_LEGACY is similar, but it allows to keep an original C++ “legacy” implementation of the pass, which is used if the compiler is not built with libswift.

include/swift/module.modulemap

@@ -3,3 +3,8 @@ module SILBridging {
   export *
 }
 
+module OptimizerBridging {
+  header "SILOptimizer/OptimizerBridging.h"
+  export *
+}
+