[analyzer] Use explicit call description mode in MIGChecker (#91331)

This commit explicitly specifies the matching mode (C library function,
any non-method function, or C++ method) for the `CallDescription`s
constructed in the checker `osx.MIG`.

The code was simplified to use a `CallDescriptionMap` instead of a raw
vector of pairs.

This change won't cause major functional changes, but isn't NFC because
it ensures that e.g. call descriptions for a non-method function won't
accidentally match a method that has the same name.

Separate commits have already performed this change in other checkers:
- easy cases: e2f1cbae,
    6d64f8e1
- MallocChecker: d6d84b5d
- iterator checkers: 06eedffe
- InvalidPtr checker: 024281d4
- apiModeling.llvm.ReturnValue: 97dd8e3c

... and follow-up commits will handle the remaining few checkers.

My goal is to ensure that the call description mode is always explicitly
specified and eliminate (or strongly restrict) the vague "may be either
a method or a simple function" mode that's the current default.