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

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"

Reproduce the Python example from the issue with mypy and trace the analyzer path that formats the base-class incompatibility diagnostics. Done means the errors no longer expose the mangled "-redefinition" name while still reporting the relevant incompatibility.