[flang][OpenMP] Change clause modifier representation in parser (#116656)

The main issue to solve is that OpenMP modifiers can be specified in any
order, so the parser cannot expect any specific modifier at a given
position. To solve that, define modifier to be a union of all allowable
specific modifiers for a given clause.

Additionally, implement modifier descriptors: for each modifier the
corresponding descriptor contains a set of properties of the modifier
that allow a common set of semantic checks. Start with the syntactic
properties defined in the spec: Required, Unique, Exclusive, Ultimate,
and implement common checks to verify each of them.

OpenMP modifier overhaul: #2/3

0 files changed, 0 insertions, 0 deletions