[dsymutil] Simplify temporary file handling.

Dsymutil's update functionality was broken on Windows because we tried
to rename a file while we're holding open handles to that file. TempFile
provides a solution for this through its keep(Twine) method. This patch
changes dsymutil to make use of that functionality.

Differential revision: https://reviews.llvm.org/D49860

llvm-svn: 338216