[MLIR] Fix MLIR vim syntax file

This commit fixes several things in the MLIR vim syntax file:
- Spell checking is now on by default only in comments.
- '#' now starts an identifier instead of starting an outline attribute
  declaration, which fixes coloring the rest of the line as a
  preprocessor directive when there is a '#' in the middle.
- '!' and '^' -prefixed identifiers are now colored as types
  and labels.

Reviewed By: bondhugula

Differential Revision: https://reviews.llvm.org/D122626