This adds a token for a forward slash to the token definition list and
the methods to `AsmParser::parseSlash()` and
`AsmParser::parseOptionalSlash()`, similar to other tokens used as
operators (e.g., star, plus, etc.). This allows implementations of
attributes that contain arithmetic expressions to support operators with
a forward slash, e.g., a division.
The newly added check tests trigger the parsing of a slash in an
attribute.
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