refactor(rustc_codegen_nvvm): parameterize build.rs over LlvmFlavor

Collapse the per-LLVM duplication in the build script into a single
`LlvmFlavor` struct with two const instances (LLVM7, LLVM19). One
`find_llvm_config`, `find_llvm_as`, `configure_libintrinsics`, and
`rustc_llvm_build` now drive both toolchain paths;
`required_major_llvm_version`, `find_llvm_config_llvm7`,
`find_llvm_config_llvm19`, and `find_llvm_as_llvm19` are gone.

Functional changes that fall out of the refactor:

- Prebuilt LLVM download now works for the `llvm19` feature too, gated on
  `USE_PREBUILT_LLVM=1` or as an automatic fallback when no LLVM 19
  toolchain is found locally. New `PREBUILT_LLVM_URL_LLVM19` points at
  the `llvm-19.1.7` release tag.
- Prebuilt download now supports `linux-x86_64` and `linux-aarch64` in
  addition to `windows-x86_64`. The "currently disabled because of
  segfaults" note on Linux x86_64 is gone — the prebuild repos that
  produce these archives have been refactored to fix the underlying
  issue.
- `PREBUILT_LLVM_URL_LLVM7` retagged to lowercase `llvm-7.1.0/` to match
  the new release-tag scheme used by the prebuild repos.
- `libintrinsics.bc` is no longer checked in; both LLVM versions now
  assemble `libintrinsics.ll` on the fly using the `llvm-as` that ships
  next to the resolved `llvm-config`. Removes the only remaining
  version-specific branch and means the LLVM 7 path can no longer drift
  silently when the `.ll` changes.

The LLVM 7 candidate-search behavior is also slightly stricter:
previously `LLVM_CONFIG` only had to literally start with "7" (matching
7, 70, 700...) and a mismatched env var skipped straight to download;
now major-version match is exact and PATH `llvm-config` is tried as a
fallback before downloading. `USE_PREBUILT_LLVM=1` still forces direct
download.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: brandonros

.github/workflows/container_images.yml

@@ -36,15 +36,15 @@ jobs:
           - name: Ubuntu-24.04/CUDA-13.0.2
             image: "rust-cuda-ubuntu24-cuda13"
             dockerfile: ./container/ubuntu24-cuda13/Dockerfile
+          - name: Ubuntu-24.04/CUDA-13.2.1/LLVM-19.1.7
+            image: "rust-cuda-ubuntu24-cuda13-llvm19"
+            dockerfile: ./container/ubuntu24-cuda13-llvm19/Dockerfile
           - name: RockyLinux-9/CUDA-12.8.1
             image: "rust-cuda-rockylinux9-cuda12"
             dockerfile: ./container/rockylinux9-cuda12/Dockerfile
           - name: RockyLinux-9/CUDA-13.0.2
             image: "rust-cuda-rockylinux9-cuda13"
             dockerfile: ./container/rockylinux9-cuda13/Dockerfile
-          - name: Ubuntu-24.04/CUDA-13.0.2/LLVM-19.1.7
-            image: "rust-cuda-ubuntu24-cuda13-llvm19"
-            dockerfile: ./container/ubuntu24-cuda13-llvm19/Dockerfile
     steps:
       - name: Free up space
         # Without this the job will likely run out of disk space.
@@ -163,12 +163,12 @@ jobs:
             image: "rust-cuda-ubuntu24-cuda12"
           - name: Ubuntu-24.04/CUDA-13.0.2
             image: "rust-cuda-ubuntu24-cuda13"
+          - name: Ubuntu-24.04/CUDA-13.2.1/LLVM-19.1.7
+            image: "rust-cuda-ubuntu24-cuda13-llvm19"
           - name: RockyLinux-9/CUDA-12.8.1
             image: "rust-cuda-rockylinux9-cuda12"
           - name: RockyLinux-9/CUDA-13.0.2
             image: "rust-cuda-rockylinux9-cuda13"
-          - name: Ubuntu-24.04/CUDA-13.0.2/LLVM-19.1.7
-            image: "rust-cuda-ubuntu24-cuda13-llvm19"
     steps:
       - name: Set lowercase repo owner
         run: echo "REPO_OWNER=$(echo ${{ github.repository_owner }} | tr A-Z a-z)" >> $GITHUB_ENV