Fix crash on invalid starred assignment target

[TheMuffinMan1320]

Fixes #20796

Root cause

*a = b is invalid Python (a bare starred assignment target must be
wrapped in a list or tuple, e.g. *a, = b). Semantic analysis
already detects this and reports:

error: Starred assignment target must be in a list or tuple

However, in analyze_lvalue() (mypy/semanal.py), the branch
handling a non-nested StarExpr lvalue only called self.fail(...)
and returned, without analyzing the inner name expression
(lval.expr).

If the right-hand side of the assignment referenced an
as-yet-undefined name, semantic analysis has to defer the statement
for a later pass. As part of that deferral, mark_incomplete()
proactively registers the starred target's name (a) in the symbol
table as a PlaceholderNode, expecting it to later be replaced with
a real definition once the lvalue is fully analyzed.

Because the non-nested-StarExpr branch never analyzed lval.expr,
that placeholder was never resolved into a real Var. Later, when
mypy tried to write the incremental cache, serialization walked the
module's symbol table and hit the leftover PlaceholderNode,
crashing with:

NotImplementedError: Cannot serialize PlaceholderNode instance

Fix

mypy/semanal.py: still analyze the inner lvalue (lval.expr) after
reporting the "Starred assignment target must be in a list or tuple"
error, matching what already happens for a valid nested star target.
This lets any pending placeholder for the name get resolved normally,
while the invalid-target error is still reported.

Test

Added testInvalidBareStarAssignmentTargetNoCrash in
test-data/unit/check-statements.test, using --debug-serialize (the
existing mechanism used by a few other crash-regression tests, e.g.
testNamedTupleDefaultValueDefer) to exercise serialization even
without writing an on-disk cache. Verified this test crashes with the
reported NotImplementedError when the fix is reverted, and passes
with the fix.

Verification

$ cat repro.py
*a = b
$ python -m mypy repro.py
repro.py:1: error: Starred assignment target must be in a list or tuple  [misc]
repro.py:1: error: Name "b" is not defined  [name-defined]
Found 2 errors in 1 file (checked 1 source file)

No more crash, and the two errors are reported as expected.

• python -m pytest -q mypy/test/testcheck.py -k check-statements — 172 passed
• python -m pytest -q mypy/test/testsemanal.py — 577 passed
• python -m mypy --config-file mypy_self_check.ini mypy/semanal.py — success

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅