Properly #include ASTConsumer.h instead of forward declaration.

Otherwise, this can lead to compile failures if a user/subclass of
FrontendAction doesn't #include this file, even if CreateASTConsumer
isn't used directly.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215353 91177308-0d34-0410-b5e6-96231b3b80d8