Use unique IL assembly names instead of automatic renaming (#85185)

Use unique IL assembly names instead of automatic renaming (#85185)

The main motivation for this is that I got stuck trying to write documentation for test merging and test suffixes. We have _il_d for IL and both _d and _cs_d for C# (and similar for _r and others - I'll use _d to denote all such tests here). Only the IL files needed to be separated into different merged groups, yet we split the C# as well.

This change renames the _il_d to just _d (unless there is already an existing _d, likely for a C# version of the test). It then replaces .assembly <name> with .assembly ASSEMBLY_NAME and adds per-configuration "prefix" IL source files with #define ASSEMBLY_NAME <name>_d. Then the rules are simple - tests have unique assembly names and no build-time renaming is required.

This lets me remove the 4 Regression merged groups with ~10 tests each in them. I rebalanced the merged groups where the number of _d tests were larger except HardwareIntrinsics (a unique directory) and Methodical (most of the tests are _d).