Add lvIsImplicitByRef information to lvaSetStruct (dotnet/coreclr#19223)

Add lvIsImplicitByRef information to lvaSetStruct (dotnet/coreclr#19223)

Before implicit byrefs were tracked by setting lvIsParam and lvIsTemp.
This change explicitly adds a flag for implicitByRef instead of overloading.
In addition, it fixes the decision to copy an implicitByRef for arm64 varargs.

Temporarily bump weight on byref params to match old behavior and avoid codegen
diffs.

Re-enabled various tests and parts of tests.

Closes dotnet/coreclr#20046
Closes dotnet/coreclr#19860

Commit migrated from https://github.com/dotnet/coreclr/commit/adf6d4661a5f7367e70256ac1c37d71149a37f5b