swift234

Try to cope with LLDB's extensions of typealiases of unbound generic type.

Type aliases of unbound generic type are ill-formed, so LLDB was
running into an assertion that the compiler would normally get
into that an extension of a generic type has already had generic
parameters provided (by bindExtensionDecl). Longer term, LLDB should
find another appropriate that doesn't rely on ill-formed ASTs. For
now, try to work around the assertion by cloning the generic parameter
list when we validate the extension.

Hopefully fixes rdar://problem/20335682, but this is untestable in the
compiler itself.

Swift SVN r26673