Fix dxc version printing and fix external validator testing

[tex3d]

Version print didn't reveal details from available version interfaces, nor did it print file version for DXIL.dll when available. We also weren't testing the external validator we build: (lib)dxil.(dll|so|dylib).

Changes:

• Clean up code a bit
• No longer skip QI for IDxcVersionInfo2 based on build, since that's for implementer, not user.
• Print CustomVersionString from IDxcVersionInfo3 if available.
• Remove ifdefs for WIN32 around file/product version calls, because that's handled in the function.
• Only print product version string if available for dxil.dll, since that's what it does for dxcompiler.dll.
• Removes old dev build detection (separate commit in PR so it's easy to reverse if desired)
• Fixes unconditional print of '(dev' as if it's an old dev build on non-Win32 targets
• Implement IDxcVersionInfo(2,3) in the IDxcCompiler wrapper interface, so it doesn't look like version 1.0
• Only print version information for external validator when selected
• Otherwise, don't print info for a validator we found that won't be used
• Fix dxc version parsing in lit.cfg
• Add built external validator testing check-extdxil
• Select the only clang/test subdirs that could even test the external validator
• Skip LitDXILValidation tests when testing compiler output using released validators

[tex3d]

Note: there are some follow-up TBDs:

• Add dedicated tests to make sure it prints what's expected when selecting validator.
• Add new option/flag to separate setting external validator path from down-level released validator testing. This way, we can test the external validator we built (across all platforms), which is currently untested.

[alsepkow]

lgtm

[bob80905]

LGTM