[pstl] Indent preprocessor directives as part of the clang-format rules

Summary:
Indenting preprocessor directives provides a significant gain in
readability. We do it for normal if statements, and it makes sense
to do it for preprocessor ifs too.

Reviewers: rodgert, MikeDvorskiy

Subscribers: jkorous, dexonsmith, jdoerfert, libcxx-commits

Differential Revision: https://reviews.llvm.org/D59767

llvm-svn: 357401