InstrProf: Remove some pointless indirection (NFC)

It doesn't make much sense to have std::unique_ptrs of std::string and
std::vector. Avoid some useless indirection by using these types
directly.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@223166 91177308-0d34-0410-b5e6-96231b3b80d8