[bugpoint] Fix "Alias must point to a definition" problems

GlobalAliases may reference function definitions, but not function declarations.

bugpoint would sometimes create invalid IR by deleting a function's body (thus
mutating a function definition into a declaration) without first 'fixing' any
GlobalAliases that reference that function definition.

This change iteratively prevents that issue. Before deleting a function's body,
it scans the module for GlobalAliases which reference that function. When
found, it eliminates them using replaceAllUsesWith.

Fixes PR20788.

Patch by Nick Johnson!

llvm-svn: 254171