cmake: Prefix Polly options with LLVM_ to avoid variable shadowing

Summary:
Before this change certain Polly variables have been used both as user-facing
CACHED cmake variables as well as uncached internal variables. Even though
this seems to have worked OK in practice, the behavior only worked due to
one variable shadowing the other. This behavior has been found confusing.
To make the use of cmake variables more clear we now prefix the cached, user
facing variables with LLVM_ as it is common habit for LLVM options and also
moved the _POLLY_ term to the beginning to ensure related options are sorted
after each other. The variables that control the behavior of LLVM/Polly are then
set by forwarding the values set in the user facing option variables.
As a result, Polly is now enabled with LLVM_POLLY_BUILD instead of BUILD_POLLY
and the linking behavior of Polly is controlled with LLVM_POLLY_LINK_INTO_TOOLS
instead of LINK_POLLY_INTO_TOOLS.

Reviewers: bogner, Meinersbur

Subscribers: pollydev, llvm-commits

Differential Revision: http://reviews.llvm.org/D19907

llvm-svn: 268537