Forbid long symbol names staring with `_`

ANGLE's shader translator prefixes symbol names with _u, but it can't do that if the symbol name is at least 1022 characters. To avoid name collisions, it has validation that if a name is at least 1022 characters and starts with _u, compilation fails.

A while ago, we made that u in _u configurable, which means the check is not correct if the compiler is invoked with anything other than u. I've also recently found a case where it makes sense to use different prefixes for different symbols, so the check would have to verify multiple prefixes, like _u, _b, etc.

I'd like to change the validation to forbid any too-long name that starts with _ like this. This requires changing the conformance2/glsl3/shader-with-1024-character-identifier.frag.html test because it does use a long name that starts with _a.

Start in src/compiler/translator/ParseContext.cpp at the linked long-name validation, then inspect conformance2/glsl3/shader-with-1024-character-identifier.frag.html and the configurable prefix in include/GLSLANG/ShaderLang.h. Update the validation and adjust the named conformance test so the expected behavior covers long names beginning with any underscore prefix.