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

llvm-svn: 369781