RFR: 8310920: Fix -Wconversion warnings in command line flags

[David Holmes]

On Mon, 26 Jun 2023 20:33:19 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

> This is broken out from but tested with the PR from https://github.com/openjdk/jdk/pull/14659

If the constraint phase is an enum, why isn't it being typed as an enum rather than char?

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PR Review: https://git.openjdk.org/jdk/pull/14665#pullrequestreview-1500050778