Materialize isbits boxed constants as device globals

[maleadt]

Since JuliaLang/julia#55045 (1.14.0-DEV.1348), small isbits unions built from
constants are kept fully boxed: codegen emits a julia.constgv slot holding a
pointer to the host box, which device code then dereferences for the payload
or type tag. relocate_gvs! used to bake that host address into the IR, so any
such kernel reads host memory → ERROR_ILLEGAL_ADDRESS.

Fixing this upstream was rejected (JuliaLang/julia#62191): inline storage is a
host-side de-optimization, and flag-guarding every eager-box site doesn't
scale. Instead, this implements @topolarity's suggestion on the GPUCompiler
side, where the GC-related objections don't apply.

relocate_gvs! now inspects the object behind each constgv slot. Non-ghost,
non-Bool isbits objects are materialized: a private constant replica of
the box (header word + payload bytes, heap-compatible layout and alignment) is
emitted into the module and the slot points at its payload. Everything else —
types, symbols, singletons, Bools, mutable objects — keeps the baked address;
those are only ever used as identity tokens.

Notes:

• The replica's header holds the same value codegen compares against in
  typeof/isa (smalltag immediate or type pointer), and === on isbits
  falls back to bits comparison, so no identity-sensitive behavior changes.
• Resolution runs before optimize!, so payload and tag loads constant-fold;
  the device code ends up identical to what inline union storage produced
  before #55045. No de-optimization, host codegen untouched.
• Every eager-boxing site funnels through literal_pointer_val_slot, so
  future upstream boxing patterns are covered automatically.
• Session-dependence marking is now precise: relocate_gvs! reports whether
  any host address survived (baked slot, or a materialized header carrying a
  non-smalltag type pointer). Kernels whose constants are all smalltag
  (Int/UInt/Char etc.) no longer get wiped from package images.

Applies to 1.13+ (older versions bake addresses in Julia itself); the union
regression only exists on 1.14.0-DEV.1348+.

[codecov]

Codecov Report

❌ Patch coverage is 95.55556% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.48%. Comparing base (1baffbb) to head (05e0d3d).

Files with missing lines	Patch %	Lines
src/jlgen.jl	95.12%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #874      +/-   ##
==========================================
+ Coverage   81.37%   81.48%   +0.10%     
==========================================
  Files          25       25              
  Lines        4769     4806      +37     
==========================================
+ Hits         3881     3916      +35     
- Misses        888      890       +2     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.