Fixes missing overflow lint for i64 #14269

The `type_overflow` lint, doesn't catch the overflow for `i64` because
the overflow happens earlier in the parse phase when the `u64` as biggest
possible int gets casted to `i64` , without checking the for overflows.
We can't lint in the parse phase, so a refactoring of the `LitInt` type
was necessary.

The types `LitInt`, `LitUint` and `LitIntUnsuffixed` where merged to one
type `LitInt` which stores it's value as `u64`. An additional parameter was
added which indicate the signedness of the type and the sign of the value.

src/librustdoc/clean/mod.rs

@@ -1947,8 +1947,6 @@ fn lit_to_string(lit: &ast::Lit) -> String {
         },
         ast::LitChar(c) => format!("'{}'", c),
         ast::LitInt(i, _t) => i.to_string(),
-        ast::LitUint(u, _t) => u.to_string(),
-        ast::LitIntUnsuffixed(i) => i.to_string(),
         ast::LitFloat(ref f, _t) => f.get().to_string(),
         ast::LitFloatUnsuffixed(ref f) => f.get().to_string(),
         ast::LitBool(b) => b.to_string(),