[clang] [clang] Bailout when the substitution of template parameter mapping is invalid. (PR #86869)

[via cfe-commits]

Sirraide wrote:

Side note: Now it’s starting to make sense why something like an unknown include was triggering this otherwise entirely unrelated failure: The first thing that the constructor of `InstantiatingTemplate` does is bail out if there was a fatal error.

https://github.com/llvm/llvm-project/pull/86869