Make __has_include a bit more resilient in the presence of macros.  <rdar://problem/12748859>.
docs: Fix mention of DefmID to MultiClassID.
TableGen/LangRef: discuss specific C-like escapes

Suggested by Joel Jones.
TableGen/LangRef: link bang operators into the productionlist

Now BangOperator should be nicely hyperlinked.

Pointed out by Joel Jones.