Explicitly represent "pseudo-generic" functions in SIL, which do

not have access to their type arguments at runtime.  Use this to
fix the emission of native thunks for imported ObjC-generic
initializers, since they may need to perform bridging.

For now, pseudo-genericity is all-or-nothing, but we may want to
make it apply only to certain type arguments.

Also, clean up some code that was using dead mangling nodes.

include/swift/Serialization/ModuleFormat.h

@@ -53,7 +53,7 @@ const uint16_t VERSION_MAJOR = 0;
 /// in source control, you should also update the comment to briefly
 /// describe what change you made. The content of this comment isn't important;
 /// it just ensures a conflict if two people change the module format.
-const uint16_t VERSION_MINOR = 250; // Last change: Add Throws bit
+const uint16_t VERSION_MINOR = 251; // Last change: SILFunctionType::isPseudogeneric
 
 using DeclID = PointerEmbeddedInt<unsigned, 31>;
 using DeclIDField = BCFixed<31>;
@@ -702,6 +702,7 @@ namespace decls_block {
     ParameterConventionField, // callee convention
     SILFunctionTypeRepresentationField, // representation
     BCFixed<1>,            // noreturn?
+    BCFixed<1>,            // pseudogeneric?
     BCFixed<1>,            // error result?
     BCFixed<30>,           // number of parameters
     BCFixed<30>,           // number of results