[flang] Fix use of __has_builtin and formatting

[clementval]

__has_builtin is not available on all compilers. Make sure it works when not defined.

Also fix some formatting issues:

• Use brace initialization where possible
• Fix wrong capitalization of variables.
• Add std:: for unit64_t and int64_t as it is mostly done in this part of the codebase.

[clementval]

Pushing this fix to unlock downstream build bots. Feel free to add post commit reviews.

[klausler]

LGTM and thanks for cleaning this up.