This PR adds semantic checks to ensure the atomic capture construct
conforms to one of the valid forms:
[capture-stmt, update-stmt], [capture-stmt, write-stmt] or [update-stmt,
capture-stmt].
Functions checkForSymbolMatch and checkForSingleVariableOnRHS are moved
from flang/lib/Lower/DirectivesCommon.h to flang/Semantics/tools.h for
reuse.
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Co-authored-by: Kiran Chandramohan <kiranchandramohan@gmail.com>
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