[code-reflection] RFR: Lower StringTemplateOp

[Mourad Abbay]

The lowering of the StringTemplateOp. We chose not to have a block for every expression body, rather blocks will be created when necessary.

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Commit messages:
 - Lower StringTemplateOp

Changes: https://git.openjdk.org/babylon/pull/39/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=39&range=00
  Stats: 106 lines in 3 files changed: 105 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/babylon/pull/39.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/39/head:pull/39

PR: https://git.openjdk.org/babylon/pull/39