Propagate hasYield from yield in ternary branches - #5904

Summary

A closure that only yields inside a ternary branch (e.g.
$x = $cond ? yield foo() : false;) was wrongly reported with
Anonymous function should return Generator<...> but return statement is missing.
even though the presence of yield anywhere in the body makes the function a
generator, so no explicit return is required.

The root cause was that the ternary expression handler did not propagate the
hasYield flag from its branches. While fixing it I found the same class of bug
in the throw expression handler.

Changes

• src/Analyser/ExprHandler/TernaryHandler.php — processExpr() now ORs
  hasYield from the condition with the results of the if and else branches
  (and the default branch of the short ternary ?:), instead of only using the
  condition's hasYield.
• tests/PHPStan/Rules/Missing/data/bug-5681.php and
  tests/PHPStan/Rules/Missing/MissingReturnRuleTest.php — regression tests.

I audited every other ExprHandler on the same axis (sub-expressions that may
contain a yield): Coalesce, Match, BinaryOp, BooleanAnd, BooleanOr,
AssignOp, Assign, Array, InterpolatedString, Isset, New,
MethodCall, StaticCall, the various call handlers and the unary handlers all
already OR hasYield from all their sub-expressions correctly. Closure and
arrow-function handlers correctly return hasYield: false (an inner closure's
yield belongs to the inner closure, not the outer function).

Root cause

MissingReturnRule treats a function as a generator (and therefore not
requiring a return) when the statement result reports hasYield. This flag is
computed bottom-up through the expression handlers. Each handler must OR the
hasYield of every sub-expression that can execute a yield. TernaryHandler
reported only the condition's hasYield.

Test

• tests/PHPStan/Rules/Missing/data/bug-5681.php reproduces the reported case
  (yield in the if branch of a ternary) plus three analogous cases: yield
  in the else branch, yield in the default branch of ?:. All three produced a false return.missing error
  before the fix and none do after. Wired up via
  MissingReturnRuleTest::testBug5681().

Fixes phpstan/phpstan#5681