[clang] [Clang] Add target triple to fix failing test (PR #104513)

[via cfe-commits]

Sirraide wrote:

> This caused a buildbot failure: https://lab.llvm.org/buildbot/#/builders/38/builds/157

That builder seems to have been using an older commit before this was merged because the `-triple x86_64-linux-gnu` is missing from the command line:
> c:\buildbot\as-builder-1\x-armv7l\build\bin\clang.exe' -cc1 -internal-isystem 'C:\buildbot\as-builder-1\x-armv7l\build\lib\clang\20\include' -nostdsysteminc 'C:\buildbot\as-builder-1\x-armv7l\llvm-project\clang\test\SemaCXX\constexpr-vectors.cpp' -Wno-uninitialized -std=c++14 -fsyntax-only -verify

https://github.com/llvm/llvm-project/pull/104513