Revert "Sema: forbid @breakpoint from being called at comptime"

This reverts commit f88b523065.

Let's please go through the language proposal process for this change. I
don't see any justification for this breaking change even in the commit
message.

src/Sema.zig

@@ -1292,7 +1292,9 @@ fn analyzeBodyInner(
                         continue;
                     },
                     .breakpoint => {
-                        try sema.zirBreakpoint(block, extended);
+                        if (!block.is_comptime) {
+                            _ = try block.addNoOp(.breakpoint);
+                        }
                         i += 1;
                         continue;
                     },
@@ -5618,13 +5620,6 @@ fn zirTrap(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!Zir.In
     return always_noreturn;
 }
 
-fn zirBreakpoint(sema: *Sema, block: *Block, extended: Zir.Inst.Extended.InstData) CompileError!void {
-    const src = LazySrcLoc.nodeOffset(@bitCast(extended.operand));
-    if (block.is_comptime)
-        return sema.fail(block, src, "encountered @breakpoint at comptime", .{});
-    _ = try block.addNoOp(.breakpoint);
-}
-
 fn zirLoop(sema: *Sema, parent_block: *Block, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
     const tracy = trace(@src());
     defer tracy.end();

test/cases/compile_errors/@trap_comptime_call.zig

@@ -0,0 +1,9 @@
+export fn entry() void {
+    comptime @trap();
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :2:14: error: encountered @trap at comptime

test/cases/compile_errors/panic_called_at_compile_time.zig

@@ -0,0 +1,13 @@
+export fn entry() void {
+    comptime {
+        @panic(
+            "aoeu",
+        );
+    }
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :3:9: error: encountered @panic at comptime

test/cases/compile_errors/runtime-only_builtins_called_at_comptime.zig

@@ -1,20 +0,0 @@
-test "comptime @panic call" {
-    comptime @panic("amogus");
-}
-
-test "comptime @trap call" {
-    comptime @trap();
-}
-
-test "comptime @breakpoint call" {
-    comptime @breakpoint();
-}
-
-// error
-// backend=stage2
-// target=native
-// is_test=true
-//
-// :2:14: error: encountered @panic at comptime
-// :6:14: error: encountered @trap at comptime
-// :10:14: error: encountered @breakpoint at comptime