[Modules] Fix misleading warning about missing textual header in umbrella header

When a textual header is present inside a umbrella dir but not in the
header, we get the misleading warning:

warning: umbrella header for module 'FooFramework' does not include
header 'Baz_Private.h'

The module map in question:

framework module FooFramework {
    umbrella header "FooUmbrella.h"

    export *
    module * { export * }

    module Private {
        textual header "Baz_Private.h"
    }
}

Fix this by taking textual headers into account.

llvm-svn: 291794