[DebugInfo] Do not emit entry values for composite locations

Summary:
This is a fix for PR45009.

When working on D67492 I made DwarfExpression emit a single
DW_OP_entry_value operation covering the whole composite location
description that is produced if a register does not have a valid DWARF
number, and is instead composed of multiple register pieces. Looking
closer at the standard, this appears to not be valid DWARF. A
DW_OP_entry_value operation's block can only be a DWARF expression or a
register location description, so it appears to not be valid for it to
hold a composite location description like that.

See DWARFv5 sec. 2.5.1.7:

"The DW_OP_entry_value operation pushes the value that the described
 location held upon entering the current subprogram. It has two
 operands: an unsigned LEB128 length, followed by a block containing a
 DWARF expression or a register location description (see Section
 2.6.1.1.3 on page 39)."

Here is a dwarf-discuss mail thread regarding this:

http://lists.dwarfstd.org...