[modules] Improve abbreviations for C++:

 * Add abbreviation for CXXMethodDecl and for FunctionProtoType. These come up
   a *lot* in C++ modules.
 * Allow typedef declarations to use the abbreviation if they're class members,
   or if they're used.

In passing, add more record name records for Clang AST node kinds.

The downside is that we had already used up our allotment of 12 abbreviations,
so this pushes us to an extra bit on each record to support the extra abbrev
kinds, which increases file size by ~1%. This patch *barely* pays for that
through the other improvements, but we've got room for another 18 abbrevs,
so we should be able to make it much more profitable with future changes.