Rename NamedDecl::getIdentifierName() to ::getNameAsCString() and make it

assert if the name is not an identifier.  Update callers to do the right
thing and avoid this method in unsafe cases.  This also fixes an objc
warning that was missing a space, and migrates a couple more to taking
IdentifierInfo and QualTypes instead of std::strings.

llvm-svn: 59936