[clang][DepScan] Allow ModuleDep to be const (llvm#132968)

This type can be exposed from C APIs, where instantiations of this type
are not expected to mutate after creation. To support this, mark the
lazy computation of build arguments mutable, as that is not intended to
otherwise mutate the state of these objects.

This was reviewed separately by @jansvoboda11

(cherry picked from commit 9aecbdf)

Author: cyndyishida

clang/include/clang/Tooling/DependencyScanning/ModuleDepCollector.h

@@ -168,7 +168,7 @@ struct ModuleDeps {
 
   /// Get (or compute) the compiler invocation that can be used to build this
   /// module. Does not include argv[0].
-  const std::vector<std::string> &getBuildArguments();
+  const std::vector<std::string> &getBuildArguments() const;
 
 private:
   friend class ModuleDepCollector;
@@ -181,7 +181,8 @@ struct ModuleDeps {
   /// including transitive dependencies.
   std::vector<std::string> FileDeps;
 
-  std::variant<std::monostate, CowCompilerInvocation, std::vector<std::string>>
+  mutable std::variant<std::monostate, CowCompilerInvocation,
+                       std::vector<std::string>>
       BuildInfo;
 };
 

clang/lib/Tooling/DependencyScanning/ModuleDepCollector.cpp

@@ -36,7 +36,9 @@ void ModuleDeps::forEachFileDep(llvm::function_ref<void(StringRef)> Cb) const {
   }
 }
 
-const std::vector<std::string> &ModuleDeps::getBuildArguments() {
+const std::vector<std::string> &ModuleDeps::getBuildArguments() const {
+  // FIXME: this operation is not thread safe and is expected to be called
+  // on a single thread. Otherwise it should be protected with a lock.
   assert(!std::holds_alternative<std::monostate>(BuildInfo) &&
          "Using uninitialized ModuleDeps");
   if (const auto *CI = std::get_if<CowCompilerInvocation>(&BuildInfo))