Parsing support for floating point types was missing a few features:
1. Parsing floating point attributes from integer literals was supported
only for types with bitwidth smaller or equal to 64.
2. Downstream users could not use `AsmParser::parseFloat` to parse float
types which are printed as integer literals.
This commit addresses both these points. It extends
`Parser::parseFloatFromIntegerLiteral` to support arbitrary bitwidth,
and exposes a new API to parse arbitrary floating point given an
fltSemantics as input. The usage of this new API is introduced in the
Test Dialect.
1e3c630fd1