[clang][docs] Register MLIR lexer for CIR docs
Register the in-tree MLIR Pygments lexer with Clang Sphinx docs so generated CIR dialect documentation can highlight fenced mlir blocks.
Also teach the MLIR lexer to consume top-level whitespace. Without this, Sphinx reports a lexer warning after the lexer is registered, and docs-clang-html fails because Sphinx warnings are treated as errors.
This makes docs-clang-html build with CLANG_ENABLE_CIR=ON.
