infra: drop llvm-19 install from setup-build-host

[jpgaribotti]

What problem does this PR solve?

• setup-build-host is pinned by SHA at every caller (today: reusable-prebuilds.yml, cpp-lint.yaml), so any change to its inline sudo ./llvm.sh 19 all step only takes effect once each caller bumps its @<sha> pin.
• The follow-up monorepo-wide LLVM 22 migration (PR QVAC-18455: bump LLVM/Clang to 22 across the monorepo via setup-llvm #1896) needs every C++ workflow to call the new setup-llvm composite action (infra: add setup-llvm composite action #1894) and bump its setup-build-host SHA pin in lockstep — otherwise the same job ends up double-installing LLVM (19 inline here + 22 from setup-llvm) for every prebuild and cpp-lint run until both pins are synced.
• Because main uses squash-merges, the only way to make those two changes land atomically is to publish the LLVM-removed setup-build-host as its own merge commit first, so the follow-up PR can pin to it.

How does it solve it?

• Removes the Linux-only Install llvm-19 step from .github/actions/setup-build-host/action.yml so the action no longer ships any Clang toolchain on its own.
• Rewords the action description to drop the "install llvm-19" claim and add an explicit callout: callers that need a versioned LLVM toolchain (every C++ workflow today) MUST invoke setup-llvm themselves on linux/windows runners; that action is the single source of truth for the LLVM major used across the monorepo.
• All other steps (Maximize disk space on Linux, Windows long-paths / CMake generator / arm64 SDK rename, base X11 dev headers + caller-supplied extras on Linux, Android NDK env + cross toolchain) are unchanged.

How was it tested?

• Verified the only callers of setup-build-host in the repo are reusable-prebuilds.yml (line 170) and cpp-lint.yaml (line 66), and both currently pin to setup-build-host@1d9b21658… — i.e. they keep installing llvm-19 inline until they bump the pin. So no in-tree consumer is broken by this commit on its own.
• The follow-up PR (QVAC-18455: bump LLVM/Clang to 22 across the monorepo via setup-llvm #1896) covers the full migration: it bumps the setup-build-host SHA pin to this commit and adds an explicit tetherto/qvac/.github/actions/setup-llvm@<sha> step to every C++ workflow, where end-to-end CI is exercised against LLVM 22.

Breaking Changes

• No behavior change for callers still pinned to the previous setup-build-host SHA — they keep installing llvm-19 inline.
• For callers that bump to this SHA: setup-build-host no longer provides any Clang/LLVM toolchain on Linux. Those callers MUST invoke tetherto/qvac/.github/actions/setup-llvm@<sha> themselves on Linux and Windows runners, otherwise bare-make generate / clang-tidy / clang-format / llvm-cov will not be available on PATH.

BEFORE:

- name: Setup build host
  uses: tetherto/qvac/.github/actions/setup-build-host@<old-sha>
  with:
    platform: linux
    arch: x64
# (LLVM/Clang implicitly installed by setup-build-host on Linux)

AFTER:

- name: Setup build host
  uses: tetherto/qvac/.github/actions/setup-build-host@<new-sha>
  with:
    platform: linux
    arch: x64

- name: Setup LLVM
  uses: tetherto/qvac/.github/actions/setup-llvm@<setup-llvm-sha>
  # defaults: version=22, windows-version=22.1.0

[github-actions]

Tier-based Approval Status

**PR Tier:** TIER1

**Current Status:** ✅ APPROVED

**Requirements:**
- 1 Team Member approval ✅ (1/1)
- 1 Team Lead OR Management approval ✅ (1/1)



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*This comment is automatically updated when reviews change.*

[gianni-cor]

/review