[code-reflection] Integrated: Simplify C99 codegen

[Gary Frost]

Some simplification of codegen by building common helpers.

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Commit messages:
 - Simplify C99 codegen

Changes: https://git.openjdk.org/babylon/pull/724/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=724&range=00
  Stats: 115 lines in 6 files changed: 64 ins; 5 del; 46 mod
  Patch: https://git.openjdk.org/babylon/pull/724.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/724/head:pull/724

PR: https://git.openjdk.org/babylon/pull/724