[flang][OpenMP] Rename OmpLoopRangeClause to OmpLooprangeClause, NFC (#170370)
The convention is to change spelling from snake_case to UpperCamel, and
use the result as a stem in derived names, e.g.
- spelling is "some_clause" -> stem is SomeClause
- spelling is "someclause" -> stem is Someclause
Member of the OmpClause variant is <stem> itself, e.g. Looprange as in
parser::OmpClause::Looprange.
Specific clause class name is Omp<stem>Clause, e.g. OmpLooprangeClause.