Add polly support to sort_includes.py

Changes:
 -  Add "isl/" as a system library prefix. Even though isl is regularly
    imported into polly, it is still used like an external library.
 -  Add "json/" as a system library prefix. Polly uses json-cpp as external
    library.
 -  Distinguish between llvm and subproject libraries. Always sort subprojects
    before LLVM. This was already the case with clang, as 'clang' comes before
    'llvm', but we also want 'polly' to be sorted before 'llvm'.

The sorting of headers that are not part of Polly or isl remains unchanged.

llvm-svn: 236929