[flang] Fix parsing bug on DATA statement

[flang] Fix parsing bug on DATA statement

This DATA statement was getting a parsing error:
`data x /a(i)%b/`

The parser was expecting the ending '/' where the '%' was. The problem
was parsing `a(i)` as a structure constructor. Instead, move the
constant subobject case before structure constructor, but match it only
if not followed by '('. That is because in `data x /a(1)(2)/`, `a(1)` is
a valid structure constructor.

Also, remove the NamedConstant alternative from DataStmtRepeat. A named
constant is always parsed as a ConstantSubobject so it can never occur.

Original-commit: flang-compiler/f18@04a76b272675d47ec7752420b15976c69a907dab
Reviewed-on: https://github.com/flang-compiler/f18/pull/1012