Use larger write sizes for MCFillFragment.
Use larger write sizes for MCFillFragment.

This brings the pr26208 testcase down to 3.2 seconds. Not checking it in
since it does create a 4GB .o file.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@258149 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit be7a41bf5d0e689c16e70e2a6b22c3c19b98f18b)

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm