Move the management of the set of conversion functions in a C++ class

into CXXRecordDecl. The only part that we do not handle this way are
using declarations, since that would require extra name lookup that we
don't currently want to pay for. This fixes <rdar://problem/8459981>,
so that LLDB can build a CXXRecordDecl and magically get all of the
right bits set.

llvm-svn: 115026