Remove temoprary files.

Previously, we created temporary files using llvm::sys::fs::createTemporaryFile
and removed them using llvm::FileRemover. This is error-prone as it is easy to
forget creating FileRemover instances after creating temporary files.
There is actually a temporary file leak bug.

This patch introduces a new class, TemporaryFile, to manage temporary files
in the RAII style.

Differential Revision: https://reviews.llvm.org/D24176

llvm-svn: 280510