[Flang][OpenMP] Allow user-defined default mappers to bypass the implicit mapper fence (#189136)

Currently we wall out implicit declare mappers from being applied to
enter/exit/update (which we'll need to address in future PRs, as this
likely should work to some extent for allocatable member mapping). A
side effect of this is that it's causing user-defined default declare
mappers to not apply in scenarios when they should.

I believe these user-defined default declare mappers should apply in all
cases where that type is mapped and no other mapper has been explicitly
specified, as they replace the original default mapping behaviour from
my admittedly shoddy specification reading skills.

The user defined default mappers should "implicitly" apply because:
1. No explicit mapper modifier is specified
2. The fallback behavior should be "as if the modifier was specified
with the default mapper-identifier" (Section 5.9)
3. The user-defined default mapper "overrides the predefined default
mapper for the given type" (Section 5.8.2)

This was 5.2-ish sectioning so information might be outdated for 6+ and
the rules might be changed (if I interpreted them correctly in the first
place).

There's a couple of other tweaks I did in the refactoring, mainly making
sure the explicit mapper can be used in conjunction with possible
specified default mappers for other types. As technically, you're
allowed to specify one mapper and map other bits and pieces in the same
mapping clause and the mapper simply doesn't apply to the other things
not of the same type.