Fix Issue 19284 - alias this not used in nested functions of a method

[RazvanN7]

When IdentifierExp(Id.This) was used, after useless extra-processing, this was resolved to the nested function context pointer. Now that ThisExp is used (and it is correct since at this point if the alias this was shadowed by a local variable, the alias this branch would not be taken), the extra processing is avoided and this is correctly resolved to the struct pointer.

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	19284	critical	alias this not used in nested functions of a method

⚠️⚠️⚠️ Warnings ⚠️⚠️⚠️

• Regression or critical bug fixes should always target the stable branch. Learn more about rebasing to stable or the D release process.

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Testing this PR locally

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

dub fetch digger
dub run digger -- build "master + dmd#8814"