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

The THEN keyword in the "ELSE IF (test) THEN" statement is useless
syntactically, and to omit it is a common error (at least for me!)
that has poor error recovery.  This patch changes the parser to
cough up a simple "expected 'THEN'" and still recognize the rest of
the IF construct.

Differential Revision: https://reviews.llvm.org/D110952