Debug Info: Support DW_AT_calling_convention on composite types.

This implements the DWARF 5 feature described at
http://www.dwarfstd.org/ShowIssue.php?issue=141215.1

This allows a consumer to understand whether a composite data type is
trivially copyable and thus should be passed by value instead of by
reference. The canonical example is being able to distinguish the
following two types:

  // S is not trivially copyable because of the explicit destructor.
  struct S {
     ~S() {}
  };

  // T is a POD type.
  struct T {
    ~T() = default;
  };

<rdar://problem/36034993>
Differential Revision: https://reviews.llvm.org/D41039

llvm-svn: 321845