[DebugInfo] Add a new DI flag to record if a C++ record is a trivial type

Summary:
This flag is used when emitting debug info and is needed to initialize subprogram and member function attributes (function options) for Codeview. These function options are used to create an accurate compiler type for UDT symbols (class/struct/union) from PDBs.

It is not easy to determine if a C++ record is trivial or not based on the current DICompositeType flags and other accessible debug information from Codeview. For example, without this flag the metadata for a non-trivial C++ record with user-defined ctor and a trivial one with a defaulted ctor are the same.

    struct S { S(); }
    struct S { S() = default; }

This change introduces a new DI flag and corresponding clang::CXXRecordDecl::isTrivial method to set the flag in the frontend.

Reviewers: rnk, zturner, llvm-commits, dblaikie, aleksandr.urakov, deadalnix

Reviewed By: rnk

Subscribers: asmith, probinson, aprantl, JDevlieghere

Differential Revision: https://reviews.llvm.org/D45122

llvm-svn: 337641