An assert message can be known only at run time; output before a failing assert is kept

[ASDAlexander77]

An assert whose message is known only at run time failed to compile:

function check(ok: boolean, what: string) {
    assert(ok, what + ": failed");   // error: operation's operand is unlinked
}

Cause: mlirGenAssert erased the op behind every message argument, as if each were a constant string, while other ops still used it.

Fix:

• A constant string is still kept as the op's text.
• Any other message becomes an optional string operand of ts.Assert (message). A message that isn't a string is cast to one.
• AssertLogic passes that operand to _assert (Windows) or __assert_fail (Unix) when the assertion fails. A null message shows the constant text instead.

assertion failed: n is 7

Tests:

• 00assert_message.ts, registered for compile, jit and the corpus: passing asserts with a concatenated message, a template literal, a null string and a number. It is clean under -mm=rc --verify-ownership.
• test-jit-assert-message-failing: a failing assert must print assertion failed: n is 7.

Second commit: output printed before a failing assert was lost. _assert and __assert_fail abort, which flushes no stream, and the JIT's _assert handler ends with _exit. So whenever stdout wasn't a console (a pipe, a file, the test runner), everything printed before the failure was lost:

print("before"); assert(false, "boom");   // piped: only "Assertion failed: boom"

Fix:

• The lowered assert now calls fflush(NULL) before _assert / __assert_fail.
• The JIT handler flushes stdout.
• The JIT binds fflush to its own CRT, as it already does puts.

test-jit-assert-message-failing now requires the output printed before the failure, then the message.

Third commit (CI): the failing-assert test. It failed on both CI platforms with Symbols not found: GC_init, GC_malloc. Run directly, tslang --emit=jit finds the GC only through the GC_LIB_PATH/TSLANG_LIB_PATH environment, which CI doesn't set. Locally the environment hid this. The test now passes --shared-libs=$<TARGET_FILE:TypeScriptRuntime>, as the other direct JIT tests do. I reproduced the failure and the fix with those variables unset.

The test is now Windows only, like the uncaught-exception JIT tests. On Linux the output and the message are both printed:

printed before
tslang: :0: Assertion `n is 7' failed.

But abort() then reaches LLVM's crash handler (Stack dump), and ctest fails the test on the signal. Catching an abort in a Linux JIT run is a separate gap.

Results (after rebasing onto main, with #389/#391/#392):

• Windows suite: 3043/3043.
• Linux suite (WSL): 3029/3029.

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