New analyzer: Ensure that mangled -redefinition names don't appear in error messages

[ilevkivskyi]

For example:

class A:
    x: str
    def x(self) -> str: ...

class B:
    x: int
    def x(self) -> int: ...

class C(A, B): ...

fails with

main:3: error: Name 'x' already defined on line 2
main:7: error: Name 'x' already defined on line 6
main:9: error: Definition of "x-redefinition" in base class "A" is incompatible with definition in base class "B"
main:9: error: Definition of "x" in base class "A" is incompatible with definition in base class "B"