Move transformations into own directory

Move all transformations into their own directory. CMakeLists are
adjusted accordingly.
(cmake) rename polly's shared lib.
Refactor Polly's Pass creation and initialization.

Rename some files and adjust cmake accordingly
Fix ABI break I made in r203587; thanks to Richard Smith for the catch.
support: add a utility function to normalise path separators

Add a utility function to convert the Windows path separator to Unix style path
separators.  This is used by a subsequent change in clang to enable the use of
Windows SDK headers on Linux.