PCH: fix a regression that reports a module is defined in both pch and pcm.
In r276159, we started to say that a module X is defined in a pch if we specify
-fmodule-name when building the pch. This caused a regression that reports
module X is defined in both pch and pcm if we generate the pch with
-fmodule-name=X and then in a separate clang invocation, we include the pch and
also import X.pcm.
This patch adds an option CompilingPCH similar to CompilingModule. When we use
-fmodule-name=X while building a pch, modular headers in X will be textually
included and the compiler knows that we are not building module X, so we don't
put module X in SUBMODULE_DEFINITION of the pch.
Differential Revision: http://reviews.llvm.org/D28415