[flang][runtime] Correct automatic parenthesized format repetition case (#71436)

In Fortran, a format automatically repeats, with a line break, until all
the data items of a data transfer statement have been consumed. PRINT
"(3I4)", 1, 2, 3, 4, 5, 6 prints two lines, for example, three values
each.

When there are nested parentheses in a format, the rightmost set of
parentheses at the top level are used for automatic repetition. PRINT
"(I4,2(I4))" 1, 2, 3, 4, 5, 6, 7 print three lines, with three values on
the first and two each on the later ones.

Fix a bug in format interpretation that causes the detection of the
"rightmost" set of parentheses to take place on each pass, leading to
problems when parentheses are even further nested.