RFR: 8313882: Fix -Wconversion warnings in runtime code [v2]

[David Holmes]

On Wed, 9 Aug 2023 20:06:58 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> I see error in some builds due to the EOF comparison though now that c is a char. I think Dean was suggesting doing the cast inside the loop.
>
> That must have been the error I was seeing but couldn't remember.  I'm putting this back to the version I tested completely.  I don't see the point of adding another local variable here and doing the checked_cast<> until we need the char to be stored.

`getc` returns an int because it has to be able to return all valid char values plus EOF.

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PR Review Comment: https://git.openjdk.org/jdk/pull/15177#discussion_r1289606432