The existing grammar was causing simple expressions to be incorrectly
parsed as a subtype indication. This change raises the precedence for
a discrete choice expression to address that issue.
Updated the test corpus to reflect the parsing correction. Additional
test cases were added for discrete choice.
This was copied from ada_mode, but results in
confusion when we have statements after the pragma
Fix allocates with if-expression, which no longer
needs double-parenthesis.