We can't place counters on function decls with no bodies. The old
behavior was to map a fresh counter to a null AST node, which is highly
suspicious at best.
For long names this is easier to read and in most cases the omitted information can be seen in the actual SIL code.
With the option -Xllvm -sil-full-demangle the old behavior can be restored.
This adds the -profile-coverage-mapping option to swift, and teaches
SILGenProfiling to generate mappings from source ranges to counters.
Swift SVN r25266