[flang][OpenMP] Added semantic checks for atomic capture, write, and update statements

This patch adds general checks for atomic read, write, and capture statements.

-check "capture statement is of the form v = x if atomic construct is read"
-check "write statement is of the form x = expr if atomic construct is write"
-check "x must not have the ALLOCATABLE attribute."
-check for non-scalar variables
-check if x (LHS variable) is accessed on the RHS of assignment statement
-improve error reporting in atomic update statemen

Reviewed By: TIFitis, raghavendhra

Differential Revision: https://reviews.llvm.org/D127620