[clang] [Clang] Fix `PPChainedCallbacks::EmbedFileNotFound()` (PR #170293)

[Nathan Corbyn via cfe-commits]

cofibrant wrote:

The issue is only reproducible in when the lexer has chained preprocessor callbacks for the following test case: [‎clang/test/Driver/mg.c](https://github.com/llvm/llvm-project/pull/170293#diff-1287a922ed2f62f4c220636ffc4fc3ffae41aa96662fd1184d69cd38b87f2e74). Could either of you suggest another flag to add to the run line to add another set of preprocessor callbacks?

https://github.com/llvm/llvm-project/pull/170293