DXC should fail gracefully when unable to allocate signature elements in available space.

Is your feature request related to a problem? Please describe.
There are some llvm_unreachable statements when a failure is detected in allocating signature elements in the available space. These are located in HLSLSignatureLower.cpp:AllocateDxilInputOutputs().

Describe the solution you'd like
Instead, the compiler should fail gracefully, without using llvm_unreachable. Perhaps emitting a diagnostic would be more desirable?

Describe alternatives you've considered
Emitting a diagnostic seems to be the most direct solution to the problem, however there doesn't seem to be any diagnostic engine in HLSignatureLower.cpp, and so it may not be an appropriate location to introduce diagnostics.

Additional context
Add any other context or screenshots about the feature request here.
https://github.com/microsoft/DirectXShaderCompiler/pull/6482#discussion_r1546706074

Start with HLSLSignatureLower.cpp, specifically AllocateDxilInputOutputs(), and review the linked pull request discussion for the existing llvm_unreachable cases. Determine how allocation failure should be reported without introducing unsupported diagnostics there. Done means the compiler fails gracefully when signature elements cannot fit, rather than reaching llvm_unreachable.