clang needs -flto when linking

[rakuco]

Consider the following excerpt:

clang++ -flto -c hello.cc
clang++ hello.o -o hello

this fails with

hello.o: file not recognized: file format not recognized

which is caused by hello.o being an LLVM IR file rather than a traditional .o file when -flto is used. It turns out that passing -flto when linking everything works fine.

A lot of software ends up working fine because they end up passing C{XX}FLAGS when linking, but that's not guaranteed.