How do I find the SourceLocation of the type definition of a VarDecl using the Clang AST?

This can be accomplished by typecasting or using the getAs() method to cast a Type class to something that has references to a type declaration.

For example,

// Get Type of variable declaration
const clang::Type* type = var_decl->getTypeSourceInfo()->getTypeLoc().getTypePtr();

// Cast it as its appropriate subtype and then obtain its declaration
// For example, if the type was a record, you can cast it as a 
// RecordType and then obtain its definition.
const clang::RecordType* record_type = type->getAs<clang::RecordType>();
if (record_type != nullptr) {
  const clang::RecordDecl* type_decl = record_type->getDecl();

  clang::SourceLocation end_of_type_decl = type_decl->getLocEnd();
}

Hope this helps!