Header guard canonicalization, clang part.

Modifications made by clang-tidy with minor tweaks.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215557 91177308-0d34-0410-b5e6-96231b3b80d8
Use std::unique_ptr to simplify memory management a bit.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215559 91177308-0d34-0410-b5e6-96231b3b80d8