[lld/mac] With -demangle, strip leading _ from non-mangled names

For

    void f();
    int main() { f(); }

`lld -demangle` now produces

    ld64.lld: error: undefined symbol: f
    >>> referenced by path/to/main.o:(symbol main+0x8)

instead of

    ld64.lld: error: undefined symbol: _f
    >>> referenced by path/to/main.o:(symbol _main+0x8)

previously. (Without `-demangle`, it still prints `_f` and `_main`.)

This does *not* match ld64's behavior, but it does match e.g. lld/COFF's
behaviour.

This is arguably easier to understand: clang prepends symbol names with `_`
on macOS, so it seems friendly if the linker removes it again in its
diagnostics. It also makes the `extern "C"` insertion diagnostics we added
recently look more self-consistent.

Differential Revision: https://reviews.llvm.org/D135189