gn build: Copy file permissions from input file in configure_file() emulation.

Most significantly, this makes bin/llvm-lit executable so that it
can be run in the usual way.

Differential Revision: https://reviews.llvm.org/D56423

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@350688 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed