ctf, ir: escape and unescape enumeration FT labels starting with `_`

ctf, ir: escape and unescape enumeration FT labels starting with `_`

This is a solution to work with TSDL 1.8 which asks identifiers starting
with `_` to be named without the `_` for viewing and analysis purposes.
Since a variant FT can refer to an enumeration FT tag which also has its
labels starting with `_`, we systematically remove leading `_` in
enumeration FT labels too.

For example:

    enum {
        _salut,
        hello,
        __zoom,
        _integer,
    } tag;

    variant <tag> {
        A _salut;
        B hello;
        C __zoom;
        D _integer;
    } var;

Once in CTF IR, the equivalent is:

    enum {
        salut,
        hello,
        _zoom,
        integer,
    } tag;

    variant <tag> {
        A salut;
        B hello;
        C _zoom;
        D integer;
    } var;

Once back to TSDL (through CTF writer), it is:

    enum {
        salut,
        hello,
        __zoom,
        _integer,
    } tag;

    variant <tag> {
        A salut;
        B hello;
        C __zoom;
        D _integer;
    } var;

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>