Temporarily try to build with PCH by default. Revert this change once we see what trouble it causes.
Temporarily try to build with PCH by default. Revert this change once
we see what trouble it causes.


git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@70017 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang