[clang] Fix implicit integer conversion for opaque enums declared in class templates (#121039)

This commit fixes issues with enumeration types instantiated from an
opaque-enum-declarations
(see [dcl.enum]) in class templates broke basic assumptions during
parsing of arithmetic
expressions due to absent (NULL TYPE) promotion types of instances of
EnumDecl.

To this end, we repeat the simple steps in `Sema::ActOnTag` to evaluate
the promotion type
of a fixed enumeration based on its underlying type (see C++11
[conv.prom] p4).

Note that if, instead, a full *enum-specifier* (subsequent curly braces)
is provided,
`Sema::ActOnEnumBody` is re-invoked on template instantiation anyway
overriding the
promotion type and hiding the issue. This is analog to how enumerations
declarations
outside of template declarations are handled.
Note that, in contrast to `Sema::ActOnEnumBody`, `Sema::ActOnTag` is
*not* called again
for the instantiated enumeration type.

Fixes #117960.

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Co-authored-by: cor3ntin <corentinjabot@gmail.com>