[clangd] Fix NDEBUG build problem introduced by rL366698

Sprinkled some #ifndef NDEBUG in Selection.cpp to make
it possible to build with NDEBUG defined.

The problem was introduced in rL366698 when using dlog
for some debug printouts. The dlog macro expands to
DEBUG_WITH_TYPE, which isn't using it's arguments in
optimized builds (when NDEBUG is defined).

llvm-svn: 367178