Minor fixes for -preview=in and targets preferring by-ref for real

[kinke]

Code compiled with -preview=in is currently supposed to link successfully against druntime/Phobos compiled without -preview=in.

There were still a few places where -preview=in makes a difference for existing in params; convert them to const [scope], analogous to PR #7593.

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[Geod24]

@kinke : Does that mean that LDC v1.24 is not compatible with -preview=in for those functions ?
As I assume it contains dlang/dmd#11828 but not this.

[kinke]

I guess so, just like DMD 2.094[.1] isn't for targets with by-ref real (Win64, 32-bit x86 IIRC). But LDC uses double-precision real for MSVC targets, so isn't as affected as DMD.