[clang] Add wasm-opt warning (PR #100321)

[Quentin Michaud via cfe-commits]

https://github.com/mh4ck-Thales created https://github.com/llvm/llvm-project/pull/100321

Add a warning when `wasm-opt` is requested with a flag (#95208) but is not found in the path.

I'm using #77148 as reference on how to add a new warning but please tell me if you can help in implementing this. Also I'm not sure in which warning group include this, at first glance none seem relevant for this specific problem. 

Regarding https://github.com/llvm/llvm-project/pull/98373 another warning might be relevant if people target WASIp2+ and try to use wasm-opt explicitly. This can be grouped in one warning "wasm-opt not available in this context" or separated in 2 for clearer error messages. WDYT?

CC @sunfishcode @sbc100 @alexcrichton 

>From 6a74d3a93d595592ee97120ad60a6c262a2c4d29 Mon Sep 17 00:00:00 2001
From: Quentin Michaud <quentin.michaud at thalesgroup.com>
Date: Wed, 24 Jul 2024 09:54:53 +0200
Subject: [PATCH] First draft of wasm-opt warning

---
 clang/lib/Driver/ToolChains/WebAssembly.cpp | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/clang/lib/Driver/ToolChains/WebAssembly.cpp b/clang/lib/Driver/ToolChains/WebAssembly.cpp
index 9aacda5fd5702..dc49fde378a9a 100644
--- a/clang/lib/Driver/ToolChains/WebAssembly.cpp
+++ b/clang/lib/Driver/ToolChains/WebAssembly.cpp
@@ -180,6 +180,9 @@ void wasm::Linker::ConstructJob(Compilation &C, const JobAction &JA,
     if (WasmOptPath == "wasm-opt") {
       WasmOptPath = {};
     }
+    if (WasmOptPath.empty()) {
+      printf("warning: wasm-opt not found but was requested\n");
+    }
   }
 
   if (!WasmOptPath.empty()) {