Build HHVM on Linux with Clang (#9129)

Summary:
This PR is an aggregation of the following PRs that fix issues in the HHVM OSS's clang build.

- #9226
- #9223
- #9220
- #9218
- #9217
- #9215
- #9214
- #9213
- #9211

It merges all the above PRs and removes `continue-on-error` for clang build.

Pull Request resolved: #9129

Test Plan: GitHub Action for the clang build should pass.

Reviewed By: alexeyt

Differential Revision: D38631930

Pulled By: Atry

fbshipit-source-id: 4fbd47ee355e760800444b7112af241555a9a8a5

.github/workflows/nix.yml

@@ -31,8 +31,6 @@ jobs:
         - hhvm_clang
         os:
         - ubuntu-latest
-    # Building hhvm_clang is expected to fail because we have not fully ported HHVM OSS to Clang yet
-    continue-on-error: ${{ matrix.package == 'hhvm_clang' }}
     runs-on: ${{matrix.os}}
     steps:
     - uses: actions/[email protected]