fix crash when constant inside comptime function has compile error

closes #625

test/compile_errors.zig

@@ -2343,4 +2343,23 @@ pub fn addCases(cases: &tests.CompileErrorContext) {
         \\pub extern fn foo(format: &const u8, ...);
     ,
         ".tmp_source.zig:2:9: error: expected type '&const u8', found '[5]u8'");
+
+    cases.add("constant inside comptime function has compile error",
+        \\const ContextAllocator = MemoryPool(usize);
+        \\
+        \\pub fn MemoryPool(comptime T: type) -> type {
+        \\    const free_list_t = @compileError("aoeu");
+        \\
+        \\    struct {
+        \\        free_list: free_list_t,
+        \\    }
+        \\}
+        \\
+        \\export fn entry() {
+        \\    var allocator: ContextAllocator = undefined;
+        \\}
+    ,
+        ".tmp_source.zig:4:25: error: aoeu",
+        ".tmp_source.zig:1:36: note: called from here",
+        ".tmp_source.zig:12:20: note: referenced here");
 }