Better demangling for improved error messages

Certain error messages have less helpful mangled function names in their output.

For example this code:

int CallMeMaybe(float, bool);

float4 main(float f : A) : SV_Target {
   return CallMeMaybe(f, false);
}

int CallMeMaybe(float f) {
    return 3;
}

produces this error:

error: External function used in non-library profile:  \01?CallMeMaybe@@YAHM_N@Z

Which is easy to gloss over and inconsistent with error messages produced elsewhere with clearer replications of the function and parameters. Either demangling this and other errors better than is currently available or getting the information from eslewhere would make them more useful.

Start by reproducing the HLSL example in the issue and tracing the diagnostic that reports the mangled external function name. Investigate the existing compiler error paths that already print clearer function signatures; done means the affected diagnostic consistently reports a readable function name and parameters, with coverage for the example and related errors.