Specs for: Add __traits(getCmdlineConstant)

[ErnyTech]

dlang/dmd#15257

[dlang-bot]

Thanks for your pull request and interest in making D better, @ErnyTech! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

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Bugzilla references

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If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

[PetarKirov]

Note: the DAutoTest build is failing:

Test the D Language specification
../dmd/generated/linux/release/64/dmd -run tools/dspec_tester.d --compiler=../dmd/generated/linux/release/64/dmd
---
// compiler defined constans: mymod.test1=some_text, mymod.test2=4

module mymod;
pragma(msg, __traits(getCmdlineConstant, "test1"));   // Prints "some_text"
pragma(msg, __traits(getCmdlineConstant, "mymod.test1")); // Prints "some_text"
pragma(msg, __traits(getCmdlineConstant, "test2", false)); // Prints string "4"
pragma(msg, __traits(getCmdlineConstant, "test2", int)); // Prints integer 4
pragma(msg, __traits(getCmdlineConstant, "test3", int, 2)); // Constant not passed to dmd --> fallback default -> Prints integer 2

---
__stdin.d(4): Error: unrecognized trait `getCmdlineConstant`
__stdin.d(4):        while evaluating `pragma(msg, __traits(getCmdlineConstant, "test1"))`
__stdin.d(5): Error: unrecognized trait `getCmdlineConstant`
__stdin.d(5):        while evaluating `pragma(msg, __traits(getCmdlineConstant, "mymod.test1"))`
__stdin.d(6): Error: unrecognized trait `getCmdlineConstant`
__stdin.d(6):        while evaluating `pragma(msg, __traits(getCmdlineConstant, "test2", false))`
__stdin.d(7): Error: unrecognized trait `getCmdlineConstant`
__stdin.d(7):        while evaluating `pragma(msg, __traits(getCmdlineConstant, "test2", int))`
__stdin.d(8): Error: unrecognized trait `getCmdlineConstant`
__stdin.d(8):        while evaluating `pragma(msg, __traits(getCmdlineConstant, "test3", int, 2))`

This is expected as the dmd PR is not yet merged. @ErnyTech please do git commit --amend --no-edit && git push -f to restart the build once the dmd PR is merged.

[dkorpel]

Closing this since dlang/dmd#15257 was closed