Fix unintentional macro shadowing in json.h (#8162)

json.h defines two lambdas with the same name
[link](https://github.com/WebAssembly/binaryen/blob/c2e47d3bffdb66e237525da24d8c28f4b5402f20/src/support/json.h#L274-L283).
From #8086, the calls in json.h are macro-expanded rather than using the
lambdas defined there. This is coincidentally ok because they have the
exact same definition, but results in an unused variable warning for the
two lambdas. This happens in #8086 and not in main because that PR adds
`#include "ir/memory-utils.h"` in wasm-interpreter.h which probably
results in simple_ast.h being included before json.h which causes the
shadowing.

[Without this
PR](https://gist.github.com/stevenfontanella/16ea5cc5144db625e5b6194706e3617d).
With this PR, compilation succeeds.

Author: stevenfontanella

src/emscripten-optimizer/simple_ast.h

@@ -310,13 +310,15 @@ struct Value {
   }
 
   char* parse(char* curr) {
-  /* space, tab, linefeed/newline, or return */
-#define is_json_space(x) (x == 32 || x == 9 || x == 10 || x == 13)
-#define skip()                                                                 \
-  {                                                                            \
-    while (*curr && is_json_space(*curr))                                      \
-      curr++;                                                                  \
-  }
+    /* space, tab, linefeed/newline, or return */
+    constexpr auto is_json_space = [](char x) {
+      return x == 32 || x == 9 || x == 10 || x == 13;
+    };
+    auto skip = [&curr, &is_json_space]() {
+      while (*curr && is_json_space(*curr)) {
+        curr++;
+      }
+    };
     skip();
     if (*curr == '"') {
       // String