llvm-cov: Simplify FunctionInstantiationSetCollector (NFC)

- Replace std::unordered_map with DenseMap
- Use std::pair instead of manually combining two unsigneds
- Assert if insert is called with invalid arguments
- Avoid an unnecessary copy of a std::vector

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218074 91177308-0d34-0410-b5e6-96231b3b80d8