[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