[flang] flang-omp-report replace std::map with llvm::DenseMap

This patch replaces the uses of std::map with llvm::DenseMap in the
flang-omp-report plugin. It also removed the 'constructClauseCount' map
due to no longer being needed after the plugin was stripped down.
This is a one of several patches focusing on switching containers from STL to LLVM's ADT library.

Reviewed By: kiranchandramohan, clementval

Differential Revision: https://reviews.llvm.org/D111977