[clang][DependencyScanning] Add API to expose module CompilerInvocation

Add a new API that returns underlying CompilerInvocation from ModuleDep.
This allows clients to inspect and modify the state inside
CompilerInvocation before turning that into build arguments.

(cherry picked from commit 9651a13)

Author: cachemeifyoucan

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

@@ -183,6 +183,11 @@ struct ModuleDeps {
   /// module. Does not include argv[0].
   const std::vector<std::string> &getBuildArguments() const;
 
+  /// Experimental: get a copy of the underlying CompilerInvocation before
+  /// calling `getBuildArguments`. This provides a short cut to inspect/modify
+  /// the compiler invocation state.
+  CowCompilerInvocation getUnderlyingCompilerInvocation() const;
+
 private:
   friend class ModuleDepCollector;
   friend class ModuleDepCollectorPP;

clang/lib/Tooling/DependencyScanning/ModuleDepCollector.cpp

@@ -46,6 +46,13 @@ const std::vector<std::string> &ModuleDeps::getBuildArguments() const {
   return std::get<std::vector<std::string>>(BuildInfo);
 }
 
+
+CowCompilerInvocation ModuleDeps::getUnderlyingCompilerInvocation() const {
+  assert(std::holds_alternative<CowCompilerInvocation>(BuildInfo) &&
+         "ModuleDeps doesn't hold compiler invocation");
+  return *std::get_if<CowCompilerInvocation>(&BuildInfo);
+}
+
 static void
 optimizeHeaderSearchOpts(HeaderSearchOptions &Opts, ASTReader &Reader,
                          const serialization::ModuleFile &MF,