[compiler-rt] list_union() is actually an intersect operation. Rename it.

Summary:
Given:
  set(T1 a b c)
  set(T2 b c d)
  message("T1=${T1}")
  message("T2=${T2}")
  list_union(T3 T1 T2)
  message("T3=${T3}")
cmake emitted:
  T1=a;b;c
  T2=b;c;d
  T3=b;c

Reviewers: beanz

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D16580

llvm-svn: 258916