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

Fixes #86757

We missed to handle the invalid case when substituting into the
parameter mapping of an constraint during normalization.
The constructor of `InstantiatingTemplate` will bail out (no
`CodeSynthesisContext` will be added to the instantiation stack) if
there was a fatal error, consequently we should stop doing any further
template instantiations.