Prevent outputting HeaderFileInfos for files not used as headers

When building an AST file, we don't want to output HeaderFileInfo
structures for files that are not actually used as headers in the
current context.  This can lead to assuming that unrelated files have
include counts of 0, defeating multiple-include prevention.

This is accomplished by adding an IsValid bit to the HFI.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203813 91177308-0d34-0410-b5e6-96231b3b80d8