Add integral-constant-expression validation for collapse, tile, cache, and gang clauses.

[Ryanpadrone]

Feature:

– This PR adds validation and verification tests for the OpenACC 3.4 clarification that introduces the term integral-constant-expression and aligns its meaning with the base languages (see §1.6).
– An integral constant expression is a compile-time constant integer expression as defined by the host language (C, C++, Fortran), and is required by the specification in several clauses and directives

What the tests validate:

The tests ensure that valid compile-time integer constants are accepted by compilers when used in positions where the OpenACC specification explicitly requires an integral constant expression, and that the associated constructs execute correctly at runtime.

Coverage includes the following spec locations:

• Collapse clause (§2.9.1)
  Argument must be a positive, non-zero integral constant expression
• Tile clause (§2.9.8)
  Each tile size must be a positive, non-zero integral-constant-expression (or *).
• Cache directive (§2.10)
  Array element indices, lower bounds, and lengths may include integral-constant-expressions.
• Gang clause – dim: argument (§2.9.2)
  The dim argument must be an integral-constant-expression evaluating to 1, 2, or 3.

Test design:

Each language file contains seven tests (T1-T6) that:

• Use language appropriate forms of ICE
  •C: integer macros and enum constants
  • C++: constexpr values and (scoped) enums
  • Fortran: integer PARAMETER expressions
• Place ICE values directly into spec-mandated positions
• perform a runtime computation check (vector add / scale) to confirm the construct executes correctly, not just parse

Test mapping

• T1: collapse clause (arithmetic ICE forms)
• T2: tile clause (single tile ICE)
• T3: tile clause (multi-dimensional ICE lists)
• T4: cache directive (ICE used in element index)
• T5: cache directive (ICE used in lower/length ranges)
• T6: gang(dim: ICE)

Notes on cache behavior

cache is a performance hint and may be ignored by implementations. The tests primarily validate syntax acceptance of integral-constant-expressions in cache subscript/range forms. In all three language versions (C, C++, and Fortran), the cache directive is placed inside the actual parallel loop body to maintain consistent structure across implementations. The cached array section or element uses integral-constant-expressions exactly where permitted by §2.10.

Expected compiler behavior and results

•NVHPC 25.5: Passes T1-T6
•GCC 15.2.0: Passes T1-T5; fails T6 (parser rejects "dim:" keyword form)
•Cray 18.0.0: Passes T1-T5 (may emit cache warnings); fails T6 (parser rejects "dim:" keyword form)

Failures in T6 occur at directive parse time, before ICE semantic checking, and indicate incomplete support for the dim: keyword syntax—not an invalid test or misuse of ICE.