[code-reflection] Integrated: HAT more consistent use of scoped code builder context + removed initialization of KernelContext in kernels

[Gary Frost]

More cleanup of C99 code gen for kernels and compute methods. 

We now pass in ScopedCodeBuildContext to both. 

We now inline map KernelContext accesses directly to appropriate methods for cuda and opencl 

So `kc.x` -> get_global_id(0) for opencl, rather than initializing a local KernelContext struct. 

This makes code gen less verbose.

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Commit messages:
 - removed kernelcontext initialization from kernels
 - more consistant use of ScopedContext

Changes: https://git.openjdk.org/babylon/pull/555/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=555&range=00
  Stats: 491 lines in 16 files changed: 104 ins; 295 del; 92 mod
  Patch: https://git.openjdk.org/babylon/pull/555.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/555/head:pull/555

PR: https://git.openjdk.org/babylon/pull/555