[flang] Better error recovery for missing THEN in IF construct

Presented with "IF (...)" with no following tokens in the statement,
diagnose a missing "THEN" instead of complaining about all of the
possible action statement initial tokens that could have been there
for a non-construct IF statement.

Fixes https://github.com/llvm/llvm-project/issues/62299.

Differential Revision: https://reviews.llvm.org/D150783