[flang] Duplicate names for ac-implied-do variables erroneously cause errors
According to section 19.4, paragraph 5, the scope of an ac-implied-do variable
is the enclosing ac-implied-do. But we were not creating new scopes upon
entry to an ac-implied-do. This was causing error messages to be erroneously
I fixed, the code, added a test to array-constr-values.f90, added the test
folding15.f90 and corrected the test symbol05.f90.
Differential Revision: https://reviews.llvm.org/D91560