Maintain type source information for functions through template

instantiation.  Based on a patch by Enea Zaffanella!  I found a way to
reduce some of the redundancy between TreeTransform's "standard"
FunctionProtoType transformation and TemplateInstantiator's override,
and I killed off the old SubstFunctionType by adding type source info
for the last cases where we were creating FunctionDecls without TSI
(at least that get passed through template instantiation).

llvm-svn: 98252