<rdar://problem/13426257> Introduce SDKSettings.plist as an input file dependency for PCH/modules.

When we're building a precompiled header or module against an SDK on
Darwin, there will be a file SDKSettings.plist in the sysroot. Since
stat()'ing every system header on which a module or PCH file depends
is performance suicide, we instead stat() just SDKSettings.plist. This
hack works well on Darwin; it's unclear how we want to handle this on
other platforms. If there is a canonical file, we should use it; if
not, we either have to take the performance hit of stat()'ing system
headers repeatedly or roll the dice by not checking anything.