[llvm-mca][RISCV] Fix llvm-mca RISCVInstrument memory leak

There was a memory leak that presented itself once the llvm-mca
tests were committed. This leak was not checked for by the pre-commit
tests. This change changes the shared_ptr to a unique_ptr to avoid
this problem.

We will know that this fix works once committed since I don't know
whether it is possible to force a lit test to use LSan. I spent the
day trying to build llvm with LSan enabled without much luck. If
anyone knows how to build llvm with LSan for the lit-tests, I am
happy to give it another try locally.

Differential Revision: https://reviews.llvm.org/D150816