Fix a temporary MDNode leak.

While emitting debug information for function forward decalrations, we
create DISubprogram objects that aran't stored in the AllSubprograms
list, and thus won't get finalized by the DIBuilder. During the DIBuilder
finalize(), the temporary MDNode allocated for the DISubprogram
Variables field gets RAUWd with a non temporary DIArray. For the forward
declarations, simply delete that temporary node before we delete the
parent node, so that it doesn't leak.