cmake: Set LINK_POLLY_INTO_TOOLS to ON (v2)

This is the second try. This time we disable this feature if no Polly checkout
is available. For this to work we need to check if tools/polly is present
early enough that our decision is known before cmake generates Config/config.h.

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

Subscribers: pollydev, llvm-commits

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

llvm-svn: 268048