Disallow hexadecimal literals in type declarations

Existing IR syntax is ambiguous in type declarations in presence of zero sizes.
In particular, `0x1` in the type size can be interpreted as either a
hexadecimal literal corresponding to 1, or as two distinct decimal literals
separated by an `x` for sizes.  Furthermore, the shape `<0xi32>` fails lexing
because it is expected to be an integer literal.

Fix the lexer to treat `0xi32` as an integer literal `0` followed by a bare
identifier `xi32` (look one character ahead and early return instead of
erroring out).

Disallow hexadecimal literals in type declarations and forcibly split the token
into multiple parts while parsing the type.  Note that the splitting trick has
been already present to separate the element type from the preceding `x`
character.

PiperOrigin-RevId: 232880373