ASTUnit: Invert the dependency of PrecompilePreambleAction on PrecompilePreambleConsumer.

Actions outlive consumers. PR16295. Found by AddressSanitizer.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@183741 91177308-0d34-0410-b5e6-96231b3b80d8