Remove boost dependency from XLS.

It is just replicating the dependency from `@rules_hdl` and depends on the `@com_github_libbacktrace` dep being a transitive dependency.

Instead, we just use the one provided by rules_hdl. For this to work, `rules_hdl_dependency_support` needs to be invoked in WORKSPACE before loading of `rules_hdl_init`. This ordering was wrong before and also, cannot happen in a .bzl which loads both together. Since workspace macros are discouraged for hybrid bzlmod mode anyway (https://bazel.build/external/migration#hybrid-mode), we remove `initialize_external.bzl` entirely.

Historical footnote: we ended up with two copies after:
1. XLS added boost for nextpnr in https://github.com/google/xls/commit/f79c1dcd5f3a574d42081208ca8cdf9162615653.
2. rules_hdl ported it in https://github.com/hdl/bazel_rules_hdl/commit/1c23debf2cc69ef62717da1085dba5ac1e66271a
3. XLS ported a patch with an implicit dependency on rules_hdl in https://github.com/google/xls/commit/6bd8019a72b3e2dda74e435b12a65746f73d91ca.

PiperOrigin-RevId: 799357631

Internal change

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/xls/pull/2914 from xlsynth:meheff/2025-08-19-block-to-verilog 7862677a11d4ab8dc1904bc90be1c8aa86e937a5
PiperOrigin-RevId: 798358079

Merge pull request #2927 from hzeller:feature-20250821-fix-deps

PiperOrigin-RevId: 798061471

Merge pull request #2905 from xlsynth:cdleary/2025-08-19-fix-internal…

Revert changes to opt pipeline for non-synthesizable nodes due to opt…

Get bytecode_emitter_test to pass when TIv2 is on by default.

PiperOrigin-RevId: 795596987

Cache type resolution result of TypeRefTypeAnnotation, so that when t…

Integrate LLVM at llvm/llvm-project@fc44a4fcd3c5

Updates LLVM usage to match
[fc44a4fcd3c5](https://github.com/llvm/llvm-project/commit/fc44a4fcd3c5)

PiperOrigin-RevId: 794807507

Merge pull request #2811 from hzeller:feature-20250808-inline-constexpr

PiperOrigin-RevId: 792780388

Migrate away from ArrayRef(std::nullopt_t)

The upstream LLVM has deprecated ArrayRef(std::nullopt_t).  This CL migrates away from that.

PiperOrigin-RevId: 792491328