rustc: Tweak Literal APIs of proc_macro

[alexcrichton]

This commit tweaks the public api of the proc_macro::Literal structure to
solve #48889, an issue about negative integers and their representation in
proc_macro tokens. Currently the compiler assumes that all integer tokens are
positive integers rather than embedding a negative sign. The negative sign is
a separate token from the integer itself.

Currently there's a function like:

impl Literal {
    pub fn i32(i: i32) -> Literal;
}

but unfortunately this doesn't work for negative integers. When called with
negative integers weird errors end up getting thrown later during the parsing
process.

This function tweaks the definitions to instead use:

impl Literal {
    pub fn i32(i: u32) -> Literal;
}

This construction makes it more clear that negative arguments are not supported.
This additionally allows literals like -128i8 where 128, the positive
integer part of this literal, isn't representable in i8. By taking an unsigned
number in each constructor we can allow constructing the minimum literal for
each signed type as well.

The final tweak of this commit is to panic in Literal::{f32, f64} if the input
number is negative. This is similar to how infinite/nan literals are handled
today (they panic) and constructing a literal should be possible by negating a
float and then separately passing in a literal.

Closes #48889

[rust-highfive]

r? @nikomatsakis

(rust_highfive has picked a reviewer for you, use r? to override)

[alexcrichton]

r? @nrc

[petrochenkov]

Looks like negative zeroes still pass the check.

[alexcrichton]

Ah oops, just found is_sign_negative which is what I was looking for earlier.

[nrc]

I'd prefer to address this by making the parser more permissive and accept negative literals - that doesn't feel like too much work and makes it easier to handle negative literals for proc macros.

[alexcrichton]

Ok sounds like this isn't the direction the discussion on #48889 is headed, so closing