Strict validator mode

[llvm-beanz]

Is your feature request related to a problem? Please describe.
The DXIL validator misses some cases of obviously wrong IR generation (see: microsoft/DirectXShaderCompiler#3706, and other issues tagged validation.

Describe the solution you'd like
Generally we can't make the validator more restrictive for older versions, but perhaps we can add an opt-in -strict option to force validation to check extra cases. This would be useful in Clang for allowing us to test the code generator under a stricter validation mode even with older shader model targets.