[flang] Improve error recovery for bad/missing construct END statements

[flang] Improve error recovery for bad/missing construct END statements

When a multi-statement construct should end with a particular END statement
like "END SELECT", and that construct's END statement is missing or
unrecognizable, the error recovery productions should not misinterpret
a program unit END statement that follows and consume it as a misspelled
construct END statement. Doing so leads to cascading errors or a failed parse.

Differential Revision: https://reviews.llvm.org/D136896