recordDecl() AST matcher

[Aaron Ballman via cfe-commits]

It turns out that the recordDecl() AST matcher doesn't match
RecordDecl objects; instead, it matches CXXRecordDecl objects. This
is... unfortunate... as it makes writing AST matchers more complicated
because of having to translate between recordDecl()/CXXRecordDecl. It
also makes it impossible to match a struct or union declaration in C
or ObjC. However, given how prevalent recordDecl()'s use is in the
wild (I'm guessing), changing it at this point would be a Bad Thing.

For people trying to write AST matchers for languages like C or ObjC,
I would like to propose adding:

structDecl()
unionDecl()
tagDecl()

These will match nicely with the existing enumDecl() AST matcher.

Additionally, I would like to add cxxRecordDecl() to match
CXXRecordDecl objects. While it duplicates the functionality exposed
by recordDecl(), it more clearly matches the intention of which AST
node it corresponds to.

Finally, I would like to undocument recordDecl() and change our
existing documentation and AST matcher uses to use
cxxRecordDecl/structDecl() instead. Maybe someday we can deprecate
recordDecl() more officially.

I'm open to other ideas if there are better ways to move forward. If
you think changing the meaning of recordDecl() is acceptable, I can
also go that route (though I would still propose adding unionDecl()
and cxxRecordDecl() in that case).

Thanks!

~Aaron