layers: Avoid using references to out-of-scope strings

[kocdemir]

Fixes possible crashes when using message filters and other string lists.

[ci-tester-lunarg]

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[artem-lunarg]

I'd like to point out that the original code is correct. The suggested change is more common though. C++ allows to bind rvalues to const reference. This often looks unusual but the reference will stay alive.

https://stackoverflow.com/questions/11560339/returning-temporary-object-and-binding-to-const-reference

[kocdemir]

Right, it should be working fine but I somehow experienced a crash on the usage of the string built this way on MacOS arm64 with the distributed SDK binaries, in an app with multiple vulkan instances spawned. I can debug a bit more if it happens again to see if there are some other underlying issues, but I believe not having references fixed the issue and makes the code more readable as well. Thanks

[ci-tester-lunarg]

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