Fix rust-lang#115150 by encoding f32 and f64 correctly for cross-language CFI. I
missed changing the encoding for f32 and f64 when I introduced the
integer normalization option in rust-lang#105452 as integer normalization does
not include floating point. `f32` and `f64` should be always encoded as
`f` and `d` since they are both FFI safe when their representation are
the same (i.e., IEEE 754) for both the Rust compiler and Clang.

…=thomcc

Name what ln_gamma does

The previous description omitted some important details.

Allow overwriting ExpnId for concurrent decoding

These assertions only hold for the single threaded compiler. They were triggered in rust-lang#115003.

…piler-errors

Fix CFI: f32 and f64 are encoded incorrectly for cross-language CFI

Fix rust-lang#115150 by encoding f32 and f64 correctly for cross-language CFI. I missed changing the encoding for f32 and f64 when I introduced the integer normalization option in rust-lang#105452 as integer normalization does not include floating point. `f32` and `f64` should be always encoded as `f` and `d` since they are both FFI safe when their representation are the same (i.e., IEEE 754) for both the Rust compiler and Clang.

…gillot

remove some unnecessary ignore-debug clauses

ignore-debug is only needed when the debug assertions *in the standard library* somehow affect the test. This can happen with inlining but otherwise should be rare. ignore-debug is problematic since PR CI is only run with debug assertions.

r? `@cjgillot` since it looks like you added most of these

… r=petrochenkov

Add comment to the push_trailing function

## Add comment to the `push_trailing` function for clarity.

I improve the explanation by describing:

- how the code handles unicode and emoji characters using `char_indices`,
- how the code handles the absence of high indexes, and
- what the code's overall aim is.