This patch fixes a crash after rebuilding call AST of
an __unknown_anytype(...). In this case, we rebuild the
vararg function type specially to convert the call expression
to  something that IRGen can handle. However, FunctionDecl
as rebuilt in RebuildUnknownAnyExpr::resolveDecl is bogus and
results in crash when accessing its params later on. This
patch fixes the crash by rebuilding the FunctionDecl to match
its new resolved type. rdar://15297105.
John McCall, please review post-commit.