MSVC PDB generation strategies

In !6721 (closed), I tried to improve CI by using flags that work with sccache. However, CMake uses an incompatible strategy for PDB files in that it uses a per-library .pdb file rather than per-TU files which just doesn't work with any (simple) caching mechanism. I propose a per-target property which can be used to select the PDB strategy for the target (names subject to bikeshedding):

set(CMAKE_PDB_STRATEGY PER_TU) # defaults to PER_TARGET for compat
set(CMAKE_PDB_LEVEL NONE) # or DEFAULT or EDIT_AND_CONTINUE
# also CMAKE_PDB_LEVEL_<CONFIG> with sensible defaults per config

AFAICT, EDIT_AND_CONTINUE is not compatible with PER_TU since it requires /ZI which is incompatible with sccache (fine for CI which doesn't need EDIT_AND_CONTINUE anyways).

Things to consider:

See CMP0091 for how to move the existing flags out of the global settings into these target properties
Behaviors with MSVC-emulating compilers (clang-cl)?
Gathering per-TU .pdb files into a final target-wide .pdb file (related: how do OBJECT libraries do this?).
Could/should PDB_LEVEL be shared at some level with other compilers (-g, -ggdb, -Og, etc.)?
Can PDB_STRATEGY also encompass -gsplit-dwarf?
What to do if the strategy doesn't support the level? Downgrade the strategy, level, or error?