swift234

[docs] Fix syntax highlighting issues to support newer Sphinxes. (#4283)

- Update the Pygments lexer we use for parsing Swift-like code.
- State more explicitly which highlighting should be used in which
  code blocks.
- Disable highlighting altogether in certain cases (such as SIL.rst,
  which has equal amounts grammar and SIL excerpts).

This should fix the warnings-as-error issues coming from Sphinx > 1.3.4.

Based on a patch by Jeremy Fergason!

https://bugs.swift.org/browse/SR-620