sessiond: ust: conditionally enable the underscore prefix variant quirk

Application contexts are expressed as variants. LTTng-UST announces
those by registering an enumeration named `..._tag`. It then registers a
variant as part of the event context that contains the various possible
types.

Unfortunately, the names used in the enumeration and variant don't
match: the enumeration names are all prefixed with an underscore while
the variant type tag fields aren't.

While the CTF 1.8.3 specification mentions that
underscores *should* (not *must*) be removed by CTF readers. Babeltrace
1.x (and possibly others) expect a perfect match between the names used
by tags and variants.

When the UNDERSCORE_PREFIXED_VARIANT_TAG_MAPPINGS quirk is enabled,
the variant's fields are modified to match the mappings of its tag.

From ABI version >= 10.x, the variant fields and tag mapping names
correctly match, making this work-around unnecessary.

However, since the variants produced by LTTng-UST contain TSDL-unsafe
names, a variant/selector sanitization pass is performed before
serializing a trace class hierarchy to TSDL.

The variant_tsdl_keyword_sanitizer visitor is used to visit field before
it is handed-over to the actual TSDL-producing visitor.

As it visits fields, the variant_tsdl_keyword_sanitizer populates a
"type_overrider" with TSDL-safe replacements for any variant or
enumeration that uses TSDL-unsafe identifiers (reserved keywords).

The type_overrider, in turn, is used by the rest of the TSDL
serialization visitor (tsdl_field_visitor) to swap any TSDL-unsafe types
with their sanitized version.

The tsdl_field_visitor owns the type_overrider and only briefly shares
it with the variant_tsdl_keyword_sanitizer which takes a reference to
it.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ib61eafc452338a99a02b9829cbd049cb6fa48ead
Depends-on: lttng-ust: Ia7e78096a9c31cd4c0574d599c961067d8f03791