[OPENMP]Allow to redefine entry for the variables definitions.

If the variable was declared and marked as declare target, a new offload
entry with size 0 is created. But if later a definition is created and
marked as declare target, this definition is not added to the entry set
and the definition remains not mapped to the target. Patch fixes this
problem allowing to redefine the size and linkage for
previously registered declaration.

llvm-svn: 355960