Change callback return type and add Doxygen comment

Sets the `PreModuleImportCallback`'s setter return type to void to
account for the lambda functions in LLDB's progress reporting not
actually using its return value. Also adds a Doxygen comment for `SetPreModuleImportCallback`

Author: chelcassanova

include/swift/AST/ASTContext.h

@@ -294,7 +294,8 @@ class ASTContext final {
   /// OutputBackend for writing outputs.
   llvm::IntrusiveRefCntPtr<llvm::vfs::OutputBackend> OutputBackend;
 
-  void SetPreModuleImportCallback(std::function<bool(llvm::StringRef ModuleName, bool IsOverlay)> callback);
+  /// Set the callback function that is invoked before Swift module importing is performed
+  void SetPreModuleImportCallback(std::function<void(llvm::StringRef ModuleName, bool IsOverlay)> callback);
 
   /// If the shared pointer is not a \c nullptr and the pointee is \c true,
   /// all operations working on this ASTContext should be aborted at the next
@@ -406,7 +407,7 @@ class ASTContext final {
   getAllocator(AllocationArena arena = AllocationArena::Permanent) const;
 
   /// An optional generic callback function invoked prior to importing a module.
-  mutable std::function<bool(llvm::StringRef ModuleName, bool IsOverlay)>
+  mutable std::function<void(llvm::StringRef ModuleName, bool IsOverlay)>
       PreModuleImportCallback;
 
 public:

lib/AST/ASTContext.cpp

@@ -714,8 +714,8 @@ ASTContext::~ASTContext() {
   getImpl().~Implementation();
 }
 
-void ASTContext::SetPreModuleImportCallback(std::function<bool(llvm::StringRef ModuleName, bool IsOverlay)> callback) {
-  ASTContext::PreModuleImportCallback = callback;
+void ASTContext::SetPreModuleImportCallback(std::function<void(llvm::StringRef ModuleName, bool IsOverlay)> callback) {
+  PreModuleImportCallback = callback;
 }
 
 llvm::BumpPtrAllocator &ASTContext::getAllocator(AllocationArena arena) const {