[gn build] (manually) port 36f01909 (llvm-debuginfod)

LLVM_ENABLE_HTTPLIB is always off in the GN build. This means the
test for this won't run, so it's not really necessary to add the
llvm-debuginfod binary (and the binary might not even be that
interesting with LLVM_ENABLE_HTTPLIB off). But maybe we want
to add support for LLVM_ENABLE_HTTPLIB at some point, so let's
add the binary too. (Usually we wouldn't, since it's not needed
to get tests to pass.)