Reduce false positive identification of C names as Dlang mangled names. This happens
when a C function uses the prefix `_D`.
The [Dlang ABI](https://dlang.org/spec/abi.html#name_mangling) shows that mangled names
have a length immediately following the `_D` prefix. This change checks for a digit
after the `_D` prefix, when identifying the mangling scheme of a symbol. This doesn't
prevent false positives entirely, but does make it less likely.
