[CMake] Set policy CMP0051 to OLD globally.

When you use generator expressions in a library sources list,
and then later access the SOURCES property, the OLD behavior
(CMake 3.0 and earlier) would not include these expressions in
the SOURCES property.  The NEW behavior (starting in CMake 3.1)
is that they do include the generator expressions in the SOURCES
property.

Differential Revision: http://reviews.llvm.org/D7870
Reviewed By: Chris Bieneman

llvm-svn: 230396