cmake: Set LINK_POLLY_INTO_TOOLS to ON

With Polly checked into LLVM it was since a long time possible to compile
clang/opt/bugpoint with Polly support directly linked in, instead of only
providing Polly as a separate loadable module. This commit switches the
default from providing Polly as a module to linking Polly into tools, such
that it becomes unnecessary to load the Polly module when playing with Polly.
Such configuration has shown a lot more convenient for day-to-day Polly use.

This change does not impact the default behavior of any tool, if Polly is not
explicitly enabled when calling clang/opt/bugpoint Polly does not affect
compilation.

This change also does not impact normal LLVM/clang checkouts that do not
contain Polly.

Reviewers: jdoerfert, Meinersbur, sebpop, etherzhhb, zinob, hiraditya

Subscribers: pollydev, llvm-commits

Differential Revision: http://reviews.llvm.org/D19711

llvm-svn: 268033