[llvm] Add new helper `Module::getRequiredFunction`. (PR #82761)

[Arthur Eubanks via llvm-commits]

aeubanks wrote:

I'm not sure this is worth adding to `Module` as opposed to just handling a null return from `getFunction()` at the callsite. 

> In the RewriteDescriptor, the presence of only a single argument to getFunction is required, thus adding a new helper function.

Why is this?

https://github.com/llvm/llvm-project/pull/82761