[Flang][flang-omp-report] Switch from std::string to StringRef (where possible)

This patch makes the following changes to flang-omp-report:
 - Update 'normalize_clause_name' parameter to use llvm::StringRef instead of
   std::sting.
 - Change usages of std::tolower to llvm::toLower from
   "ADT/StringExtras.h".
This is a one of several patches focusing on switching containers from STL to LLVM's ADT library.

Reviewed By: Leporacanthicus, clementval

Differential Revision: https://reviews.llvm.org/D111980