spirv-val: Break up the 3000 line ValidateExtInst function (#6364)

This is 2 commits, first just move the `get_ext_inst_name` lambda to be
a function so it doesn't have to be passed around everywhere. Then I
just created dedicated `ValidateExtInst*` for each set of extended
instruction sets currently supported

This is just a copy-and-paste job, but shows a LOT of diff because it
removed a level of indentation from everything

Author: spencer-lunarg