[SE] Add .clang-format

Summary:
The .clang-tidy file is copied from the top-level LLVM source directory.

Also fix warnings generated by clang-format:

* Moved SimpleHostPlatformDevice.h so its header include guard could
  have the right format.
* Changed signatures of methods taking llvm::Twine by value to take it
  by const ref instead.
* Add "noexcept" to some move constructors and assignment operators.
* Removed a bunch of places where single-statement loops and
  conditionals were surrounded with braces. (This was not found by the
  current clang-tidy, but with a local patch that I hope to upstream
  soon.)

Reviewers: jlebar, jprice

Subscribers: parallel_libs-commits

Differential Revision: https://reviews.llvm.org/D24468

llvm-svn: 281374