[flang] Fix shared library builds for lib/Lower.

Summary:
This adds missing definitions for functions in the Lower directory
that were causing failures in shared library builds.
The definitions for these are taken from the fir-dev branch on github.

Reviewers: sscalpone, schweitz, jeanPerier, klausler

Reviewed By: schweitz

Subscribers: mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D83771