swift234

[serialization] Don't try to cross-reference generic params of SIL functions.

SIL functions use AST GenericParamTypeDecls, but they don't have a useful
DeclContext, so they just use the AST module associated with the current
SILModule. However, when it comes time to reserialize referenced functions
with shared_external linkage (such as closures defined in fragile public
functions), the serializer was trying to cross-reference those generic
parameters rather than reserialize them, because they aren't part of the
current source file. And because these decls aren't attached to a specific
AST DeclContext, we can't properly cross-reference them---nor should we.

This commit introduces a targeted case in the cross-reference logic to force
re-serializing these declarations. In the long run we may want to reconsider
using AST GenericParamLists for SILFunctions.

rdar://problem/18673024

Swift SVN r22800