[lit] Only gather redirected files for command failures.
[lit] Only gather redirected files for command failures.

 - The intended use of this was just in diagnostics, so we shouldn't pay the
   cost of reading these all the time.

 - This will avoid including the full output of each command in tests which
   fail, but the most important use case for this was to gather the output of
   the specific command which failed.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@272365 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm