Don't build identifiers for C++ constructors, destructors, or

conversion functions. Instead, we just use a placeholder identifier
for these (e.g., "<constructor>") and override NamedDecl::getName() to
provide a human-readable name.

This is one potential solution to the problem; another solution would
be to replace the use of IdentifierInfo* in NamedDecl with a different
class that deals with identifiers better. I'm also prototyping that to
see how it compares, but this commit is better than what we had
previously.

llvm-svn: 59193