ERR/E5.1: lambda-specific raise-not-failable hint

A closure literal whose body raises but is annotated non-failable (or has
no ! in its return) now gets a lambda-specific diagnostic telling the user
to declare the failable return explicitly, instead of the generic "raise
is only valid inside a failable function". Failability is never inferred
for a lambda, so a raising lambda with no ! is a hard error that should
point at the fix.

New in_lambda_body flag (save/restore for nesting) set around the lambda
body lowering in lowerLambda; diagRaiseNotFailable branches on it.
Top-level functions keep the generic message.

Test: 1043-errors-lambda-raise-annotation-hint.sx.

examples/expected/1043-errors-lambda-raise-annotation-hint.exit

@@ -0,0 +1 @@
+1

examples/expected/1043-errors-lambda-raise-annotation-hint.stderr

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+error: lambda body raises; declare its return type explicitly with `-> (T, !)` or `-> (T, !Named)`
+  --> /Users/agra/projects/sx/examples/1043-errors-lambda-raise-annotation-hint.sx:18:61
+   |
+18 |     print("{}\n", take(closure((x: s32) -> s32 { if x < 0 { raise error.Neg; } return x; }), -1));
+   |                                                             ^^^^^^^^^^^^^^^^

examples/expected/1043-errors-lambda-raise-annotation-hint.stdout

@@ -0,0 +1 @@
+