[clang-tools-extra] [clangd] Support .clangd command line modifications for C++ modules (PR #122606)

[Petr Polezhaev via cfe-commits]

================
@@ -32,6 +32,9 @@ namespace {
 /// interfere with each other.
 class ModuleDependencyScanner {
 public:
+  using CommandProvider =
----------------
petr-polezhaev wrote:

In my original approach I just made the original CommandMangler (a struct) shared_ptr and passed it here. It is also safer than to capture a reference since that's the only way how I can pass it inside the ProjectModules with the unique_function. I don't know why it's converted to a function object. Should I do it this way (shared_ptr) or keep the function thing?

It would be basically
```
virtual void setCommandMangler(std::shared_ptr<CommandMangler> Mangler);
```

https://github.com/llvm/llvm-project/pull/122606