[IR] Drop const in DILocation::getMergedLocation

This commit removes constness from DILocation::getMergedLocation and
fixes all its users accordingly.

Having constness on the parameters forced the return type to be const
as well, which does force usage of `const_cast` when the location needs
to be used in metadata nodes.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D149942