[Flang] Ported test_symbols to Python

Due to unavailability of Flang testing on Windows, the shell scripts
are being ported to Python. The following changes are being made in
this patch: removed test_symbols.sh and common.sh, and ported them
to Python. Changes to the tests themselves so that they use the
python scripts instead of the shell script.

Reviewed By: Meinersbur, awarzynski, kiranchandramohan

Differential Revision: https://reviews.llvm.org/D107041