[Chapel Merge] Fix an assertion error that comes up when cobegins

Branch: refs/heads/master
Revision: f8694cb
Author: bradcray
Log Message:

Merge pull request #16815 from bradcray/make-cobegin-resilient-to-syntax-errors

Fix an assertion error that comes up when cobegins contain syntax errors

[reviewed by @daviditen ]

This fixes an assertion error pointed out in an SO issue
that occurs when the body of a ‘cobegin’ contains a syntax error.
Here, I’m working around it by replacing the assertion with a test and
returning early in such cases, believing the compiler will exit before
the lack of a built cobegin statement catches up with us.

Modified Files:
A test/parallel/cobegin/errors/cobeginSyntaxError.chpl
A test/parallel/cobegin/errors/cobeginSyntaxError.good
M compiler/AST/build.cpp

Compare: https://github.com/chapel-lang/chapel/compare/e2571d8b8088...f8694cb09812