[FileCheck] Document FILECHECK_OPTS in -help
[FileCheck] Document FILECHECK_OPTS in -help

Reviewed By: thopre

Differential Revision: https://reviews.llvm.org/D65707

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@368787 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm