When enable bridging header auto chaining, it is possible for the
compilation to have a PCH file input for the bridging header from a
binary swift module dependency. In this case, we should not report a
bridging header for current module as bridging header can be leaking out
through swiftinterface file.
To fully distinguish the PCH files passed in through different
situation, here are the situations:
* If no chaining is used, only `-import-objc-header` option is used and
it can be used to pass either a header file or a PCH file depending if
GeneratePCH job is requested or not.
* If chaining is enabled, `-import-objc-header` is only used to pass the
header file and `-import-pch` is used to pass PCH file. Chaining mode
requires PCH generation if bridging header is used.
rdar://144623388
Add ability to automatically chaining the bridging headers discovered from all
dependencies module when doing swift caching build. This will eliminate all
implicit bridging header imports from the build and make the bridging header
importing behavior much more reliable, while keep the compatibility at maximum.
For example, if the current module A depends on module B and C, and both B and
C are binary modules that uses bridging header, when building module A,
dependency scanner will construct a new header that chains three bridging
headers together with the option to build a PCH from it. This will make all
importing errors more obvious while improving the performance.
