[llvm] Compute GUIDs once and store in metadata (PR #184065)

[Mircea Trofin via llvm-commits]

================
@@ -576,7 +577,22 @@ class LLVM_ABI Module {
   // Use global_size() to get the total number of global variables.
   // Use globals() to get the range of all global variables.
 
+  std::optional<GlobalValue::GUID> getGUID(const Value* V) const {
+    const auto It = ValueToGUIDMap.find(V);
+    if (It == ValueToGUIDMap.end()) {
+      return {};
+    }
+
+    return It->getSecond();
+  }
+
+  void insertGUID(const Value* V, GlobalValue::GUID GUID) {
+    ValueToGUIDMap[V] = GUID;
----------------
mtrofin wrote:

do we want to 

```
auto [_, inserted] = ValueToGUIDMap.insert({V, GUID})
(void)inserted;
assert(inserted && "expected to only insert once")
```

(or if there's a reason for repeated calls, we can assert the present value has same GUID)

https://github.com/llvm/llvm-project/pull/184065