[Analyzer] Allow creation of stack frame for functions without definition

Retrieving the parameter location of functions was disabled because it
may causes crashes due to the fact that functions may have multiple
declarations and without definition it is difficult to ensure that
always the same declration is used. Now parameters are stored in
`ParamRegions` which are independent of the declaration of the function,
therefore the same parameters always have the same regions,
independently of the function declaration used actually. This allows us
to remove the limitation described above.

Differential Revision: https://reviews.llvm.org/D80286