[gnrt] Consolidate how conditional compilation expressions are handled.

Overview
--------

Rust supports conditional compilation.  This is typically encountered in
`#[cfg(...)]` attributes _within_ a crate, but _cross-crate_
dependencies or crate features can also be conditionally enabled.

Before this CL, conditionally compilation expression were handled in two
places:

* `platform.rs` handled removing unnecessary terms + figuring if the
  condition can ever be true on any Chromium-supported platform.
* `gn.rs` handled translating the expressions into GN syntax

After this CL, `platform.rs` is removed and the logic is consolidated
into `gn.rs`.  This is mostly enabled by preserving the `AlwaysTrue`,
`AlwaysFalse`, `Ignored`, conditional `Expr`, `Unsupported` until the
last possible moment when we need to convert a condition into GN syntax.

Motivation
----------

This CL helps to fix some discrepancies between `platform.rs`
and `gn.rs`:

* Before this CL `platform.rs` knew about `target_env` but `gn.rs`
  didn't
* Before this CL `platform.rs` knew about `windows_raw_dylib` but
  `gn.rs` didn't

Additionally, the unified/consolidated approach makes the `guppy`
migration easier in the follow-up CL at https://crrev.com/c/6259145.

Testing
-------

This CL has been tested by:

* Running `tools/crates/run_gnrt.py gen` and verifying it doesn't make
  any unexpected changes under `//third_party/rust`.  (Only `gen`
  depends on `fn collect_dependencies` from `deps.rs`.)
* Same for `tools/rust/gnrt_stdlib.py`. Note that `gnrt gen` covers all
  platforms and therefore there should be no^H^Hminimal risk from
  running `gnrt_stdlib.py` only "on my machine" (and not on
  `mac_upload_clang_arm` for example).
* Running `cargo test` under `tools/crates/gnrt`

Bug: 395924069
Change-Id: I8fbf3c5de69dc4bd684cb027954650530e2ff2c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6300716
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1427898}