Canonicalize template specialization types to prevent duplicate definitions.

[copybara-service]

Canonicalize template specialization types to prevent duplicate definitions.

Crubit wraps template specializations (such as Option<T>, Result<T, E>, Vec<T>, and tuples) in #ifndef preprocessor guards to prevent duplicate definition errors in Clang when multiple generated headers are included in the same translation unit. We ran into a chain of two issues with how these guards and types are generated:

1. include guards based on C++ type spellings
   • Problem: Previously, #ifndef guard names were derived from the formatted C++ type string. This was vulnerable to duplicate definition errors because two spellings of a C++ type could actually refer to the same underlying type but generate different guard macro names. When multiple headers were included together, the guards wouldn't interfere, Clang would desugar the aliases and canonicalize them to the same underlying C++ type, and then fail with redefinition errors.
   • Fix: Derive the #ifndef guard directly from the fully canonicalized Rust type (specialization_guard_name) using rustc's with_no_visible_paths, with_no_trimmed_paths, and with_resolve_crate_name flags to see through all pub use aliases and re-exports.
2. usize vs u64 specialization clashes
   • Problem: Basing the #ifndef guard on the Rust type meant Result<usize, ...> and Result<u64, ...> generated different guard names (..._usize_... vs ..._u64_...), since in Rust they are distinct types. However, commit 7ba4d1d previously changed C++ formatting (format_ty_for_cc) to emit uint64_t for usize in template arguments. This caused Clang to receive two identical C++ specialization definitions for Result<uint64_t, ...> guarded by different macro names, resulting in redefinition errors.
   • Fix: Canonicalize the Rust Ty upfront using a TypeFolder (canonicalize_specialization_ty) that rewrites usize/isize into platform fixed-width integers (u64/i64) and lifetimes to 'static. This provides a single source of truth for in-memory deduplication, include guard generation, and C++ code emission, allowing us to clean up the ad-hoc TypeLocation::TemplateArg formatting checks in format_type.rs.

Note that some of the diffs look completely wrong, but aren't. For example, result_cc_api.h has some diffs that appear to swap the Ok and Err types of Result, e.g. Result<isize, i8> -> Result<i8, isize>. But this is actually because the original Rust file used both, and the tool decided to swap the order they're emitted in and the diff tool doesn't realize the full block was swapped because they're so similar.

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