Simplify the scheme used for keywords, and change the classification

Simplify the scheme used for keywords, and change the classification
scheme to be more useful.

The new scheme introduces a set of categories that should be more
readable, and also reflects what we want to consider as an extension
more accurately.  Specifically, it makes the "what is a keyword"
determination accurately reflect whether the keyword is a GNU or
Microsoft extension.

I also introduced separate flags for keyword aliases; this is useful
because the classification of the aliases is mostly unrelated to the
classification of the original keyword.

This patch treats anything that's in the implementation
namespace (prefixed with "__", or "_X" where "X" is any upper-case
letter) as a keyword without marking it as an extension.  This is
consistent with the standards in that an implementation is allowed to define
arbitrary extensions in the implementation namespace without violating
the standard. This gets rid of all the nasty "extension used" warnings
for stuff like __attribute__ in -pedantic mode.  We still warn for
extensions outside of the the implementation namespace, like typeof.
If someone wants to implement -Wextensions or something like that, we
could add additional information to the keyword table.

This also removes processing for the unused "Boolean" language option;
such an extension isn't supported on any other C implementation, so I
don't see any point to adding it.

The changes to test/CodeGen/inline.c are required because previously, we
weren't actually disabling the "inline" keyword in -std=c89 mode.

I'll remove Boolean and NoExtensions from LangOptions in a follow-up
commit.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@70281 91177308-0d34-0410-b5e6-96231b3b80d8