Fix Issue 21159 - DWARF: DW_AT_pure should be emitted for pure functions

[Luhrel]

A subprogram entry may have a DW_AT_pure attribute, which is a flag. The
attribute indicates whether the subroutine was declared with the “pure”
keyword or property.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21159	enhancement	DWARF: DW_AT_pure should be emitted for pure functions

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#11616"

[thewilsonator]

This seems to not work on freebsd.

[Luhrel]

Objdump is too old on the FreeBSD machines.

Unknown AT value: 67: 1

[Geod24]

@braddr : Any chance we could get some of those machines upgraded ? Not only the FreeBSD, but the auto-tester in general. AFAIK it's been quite a while.
@jmdavis : You are the only FreeBSD user I know, any advice / previous experience with this ?

[nordlow]

Are there any functional benefits to this other than better diagnostics for tools such as gdb that can read and present dwarf information?

[Luhrel]

Are there any functional benefits to this other than better diagnostics for tools such as gdb that can read and present dwarf information?

Nope. Real benefits depend on the dwarf consumer.