[Flang] flang-omp-report replace std::vector's with llvm::SmallVector

This patch replaces all uses of std::vector with llvm::SmallVector in the flang-omp-report plugin.
This is a one of several patches focusing on switching containers from STL to LLVM's ADT library.

Reviewed By: Leporacanthicus

Differential Revision: https://reviews.llvm.org/D111709