[ opt ] Use getBodyUnraised instead of getBody.

If we are only interested in the existence of a body or looking just at
its head symbol, we need not adjust the de Bruijn indices.
[ haddock ] Properly placed comments for getBody and getBodyUnraised.
[ fix ] Removed faulty optimization for getBody.

Was used only in Epic.Injection, so the bug remained undiscovered.
[ refactor ] Removed duplicate getBody in clause compiler.
Preserve ArgInfo.  Removed last uses of defaultArg in clause compiler.

This fixes a potential issue like 1186.
Merge branch 'maint-2.4.0'