Fix Issue 20831 - __traits(getAttributes) failes to compile when used…

[BorisCarvajal]

… on a parameter with no name

I'm marking each Parameter that is actually a function parameter with STC.parameter, thus we can test if a Parameter instance is a function argument in a more robust way.
I've a couple of patchs for other issues using this approach.

An alternative fix is to generate identifiers for unnamed parameters in semantic 1, they are generated in semantic 3 which causes a few bugs now. I'll report them sometime.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20831	normal	__traits(getAttributes) failes to compile when used on a parameter with no name

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#11284"