[clang] [clang] Fix missing installed header (PR #95979)

[Ben Boeckel via cfe-commits]

mathstuf wrote:

> Nice find. That's kind of horrifying.

Indeed. CMake's old "guess the extension" behavior was always a…fun thing to deal with. Alas, this emergent behavior was unknown at the time and a straight fix is likely to break other projects :( . Plus it's not likely we'll get patched releases for the past 18 releases. Luckily, there is a fix here. We'll get this in the test suite and properly nailed down though; hopefully we can detect this behavior so that other projects can discover such instances as well.

https://github.com/llvm/llvm-project/pull/95979