Split out of D60808.
When growing a body on a body farm, it's essential to use the same redeclaration of the function that's going to be used during analysis. Otherwise our ParmVarDecls won't match the ones that are used to identify argument regions. This boils down to trusting the reasoning in AnalysisDeclContext. We shouldn't canonicalize the declaration before farming the body because it makes us not obey the sophisticated decision-making process of AnalysisDeclContext.