[flang] Inhibit case of false tokenization of Hollerith (#79029)

https://github.com/llvm/llvm-project/issues/78927 contains a case of
fixed-form source in which a Hollerith literal is mistakenly tokenized,
leading to grief later due to apparently unbalanced parentheses.

The source looks like "REAL*8 R8HEAP(SCRSIZE)" and the Hollerith literal
is misrecognized as such because it follows "8R". In order to properly
tokenize Hollerith literals in old comma-free FORMAT statements like "1
FORMAT(3I5HFLANG)", the tokenizer in the prescanner treats a letter
after an integer token ("3I") as a special case. The fix is to do this
only when the characters involved are nested in parentheses and
Hollerith is a possibility.

Fixes https://github.com/llvm/llvm-project/issues/78927.