write: make it easier to generate valid DWARF

[philipc]

A couple of issues were encountered in #448 and rust-lang/rustc_codegen_cranelift#978:

• subprograms need a DW_AT_name attribute
• DW_AT_high_pc can only be a length for DWARF version >= 4
• base addresses in location lists are new in DWARF version 3

And there'll be a whole bunch of other things that the standard requires or that are added in newer versions which we don't check for at all.

Some ideas for how to help with this:

• add these sorts of checks to the dwarf-validate example
• add a validate method to UnitTable or Unit

Ideally the API would disallow these sorts of errors, but it is too low level for this. Maybe we should provide a higher level API too.

[philipc]

This also needs to handle GNU extensions that were added to newer versions. We should emit the GNU extension for older versions, and the standard value for newer versions.

[bjorn3]

Things that would be nice to check:

• Attributes present, but not allowed either in the specified DWARF version or any version. Eg using DW_AT_high_pc witha length for DWARF version < 4
• Attributed missing, but mandatory, like DW_TAG_subprogram without DW_AT_name or specified code location.
• Attributes missing and not mandatory but likely wanted, like DW_AT_stmt_list for DW_TAG_compilation_unit when there is a .debug_line section.

Maybe we should provide a higher level API too.

That would be nice.