Debug Info: Support for DW_AT_export_symbols for anonymous structs

Debug Info: Support for DW_AT_export_symbols for anonymous structs

This implements the DWARF 5 feature described in:

http://dwarfstd.org/ShowIssue.php?issue=141212.1

To support recognizing anonymous structs:

  struct A {
    struct { // Anonymous struct
        int y;
    };
  }   a;

This patch adds a new (DI)flag to LLVM metadata:

ExportSymbols

Differential Revision: https://reviews.llvm.org/D66352

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@369781 91177308-0d34-0410-b5e6-96231b3b80d8