This reapplies D88384 with the minor modification that an assertion was
changed to a regular conditional and graceful exit from
ASTContext::mergeTypes.
Especially for templates we need to check at some point if the base
function matches the specialization we might call instead. Before this
lead to the replacement of `std::sqrt(int(2))` calls with one that
converts the argument to a `std::complex<int>`, clearly not the desired
behavior.
Reported as PR47655
Reviewed By: JonChesterfield
Differential Revision: https://reviews.llvm.org/D88384