checker: relax the "unreachable code after a @[noreturn] call" error to a warning to reduce prototyping friction (#25173)

Author: spytheman

vlib/v/checker/return.v

@@ -328,7 +328,7 @@ fn (mut c Checker) find_unreachable_statements_after_noreturn_calls(stmts []ast.
 		if stmt is ast.ExprStmt {
 			if stmt.expr is ast.CallExpr {
 				if prev_stmt_was_noreturn_call {
-					c.error('unreachable code after a @[noreturn] call', stmt.pos)
+					c.warn('unreachable code after a @[noreturn] call', stmt.pos)
 					return
 				}
 				prev_stmt_was_noreturn_call = stmt.expr.is_noreturn

vlib/v/checker/tests/noreturn_with_non_empty_loop_at_end.out

@@ -1,13 +1,13 @@
+vlib/v/checker/tests/noreturn_with_non_empty_loop_at_end.vv:18:2: warning: unreachable code after a @[noreturn] call
+   16 |     eprintln('start')
+   17 |     abc()
+   18 |     eprintln('done')
+      |     ~~~~~~~~~~~~~~~~
+   19 | }
 vlib/v/checker/tests/noreturn_with_non_empty_loop_at_end.vv:4:6: error: @[noreturn] functions should end with a call to another @[noreturn] function, or with an infinite `for {}` loop
     2 | fn another() {
     3 |     eprintln(@FN)
     4 |     for {
       |         ^
     5 |         break
     6 |     }
-vlib/v/checker/tests/noreturn_with_non_empty_loop_at_end.vv:18:2: error: unreachable code after a @[noreturn] call
-   16 |     eprintln('start')
-   17 |     abc()
-   18 |     eprintln('done')
-      |     ~~~~~~~~~~~~~~~~
-   19 | }

vlib/v/checker/tests/noreturn_with_return.out

@@ -1,3 +1,9 @@
+vlib/v/checker/tests/noreturn_with_return.vv:18:2: warning: unreachable code after a @[noreturn] call
+   16 |     eprintln('start')
+   17 |     abc()
+   18 |     eprintln('done')
+      |     ~~~~~~~~~~~~~~~~
+   19 | }
 vlib/v/checker/tests/noreturn_with_return.vv:2:1: error: [noreturn] functions cannot use return statements
     1 | @[noreturn]
     2 | fn another() {
@@ -11,9 +17,3 @@ vlib/v/checker/tests/noreturn_with_return.vv:6:2: error: @[noreturn] functions s
       |     ~~~~~~
     7 | }
     8 |
-vlib/v/checker/tests/noreturn_with_return.vv:18:2: error: unreachable code after a @[noreturn] call
-   16 |     eprintln('start')
-   17 |     abc()
-   18 |     eprintln('done')
-      |     ~~~~~~~~~~~~~~~~
-   19 | }

vlib/v/checker/tests/noreturn_without_loop_or_another_noreturn_at_end.out

@@ -1,13 +1,13 @@
+vlib/v/checker/tests/noreturn_without_loop_or_another_noreturn_at_end.vv:15:2: warning: unreachable code after a @[noreturn] call
+   13 |     eprintln('start')
+   14 |     abc()
+   15 |     eprintln('done')
+      |     ~~~~~~~~~~~~~~~~
+   16 | }
 vlib/v/checker/tests/noreturn_without_loop_or_another_noreturn_at_end.vv:3:2: error: @[noreturn] functions should end with a call to another @[noreturn] function, or with an infinite `for {}` loop
     1 | @[noreturn]
     2 | fn another() {
     3 |     eprintln(@FN)
       |     ~~~~~~~~~~~~~
     4 | }
     5 |
-vlib/v/checker/tests/noreturn_without_loop_or_another_noreturn_at_end.vv:15:2: error: unreachable code after a @[noreturn] call
-   13 |     eprintln('start')
-   14 |     abc()
-   15 |     eprintln('done')
-      |     ~~~~~~~~~~~~~~~~
-   16 | }