When writing out a PCM, we skip serializing headers' `HeaderFileInfo`
struct whenever this condition evaluates to `true`:
```c++
!HFI || (HFI->isModuleHeader && !HFI->isCompilingModuleHeader)
```
However, when Clang parses a module map file, each textual header gets a
`HFI` with `isModuleHeader=false`, `isTextualModuleHeader=true` and
`isCompilingModuleHeader=false`. This means the condition evaluates to
`false` even if the header was never included and the module map did not
affect the compilation. Each PCM file that happened to parse such module
map then contains a copy of the `HeaderFileInfo` struct for all textual
headers, and considers the containing module map affecting.
This patch makes it so that we skip headers that have not been included,
essentially removing the virality of textual headers when it comes to
PCM serialization.
