Merge some of the PCH object support with modular codegen

Merge some of the PCH object support with modular codegen

I was trying to pick this up a bit when reviewing D48426 (& perhaps D69778) - in any case, looks like D48426 added a module level flag that might not be needed.

The D48426 implementation worked by setting a module level flag, then code generating contents from the PCH a special case in ASTContext::DeclMustBeEmitted would be used to delay emitting the definition of these functions if they came from a Module with this flag.

This strategy is similar to the one initially implemented for modular codegen that was removed in D29901 in favor of the modular decls list and a bit on each decl to specify whether it's homed to a module.

One major difference between PCH object support and modular code generation, other than the specific list of decls that are homed, is the compilation model: MSVC PCH modules are built into the object file for some other source file (when compiling that source file /Yc is specified to say "this compilation is where the PCH is homed"), whereas modular code generation invokes a separate compilation for the PCH alone. So the current modular code generation test of to decide if a decl should be emitted "is the module where this decl is serialized the current main file" has to be extended (as Lubos did in D69778) to also test the command line flag -building-pch-with-obj.

Otherwise the whole thing is basically streamlined down to the modular code generation path.

This even offers one extra material improvement compared to the existing divergent implementation: Homed functions are not emitted into object files that use the pch. Instead at -O0 they are not emitted into the IR at all, and at -O1 they are emitted using available_externally (existing functionality implemented for modular code generation). The pch-codegen test has been updated to reflect this new behavior.

[If possible: I'd love it if we could not have the extra MSVC-style way of accessing dllexport-pch-homing, and just do it the modular codegen way, but I understand that it might be a limitation of existing build systems. @hans / @thakis: Do either of you know if it'd be practical to move to something more similar to .pcm handling, where the pch itself is passed to the compilation, rather than homed as a side effect of compiling some other source file?]

Reviewers: llunak, hans

Differential Revision: https://reviews.llvm.org/D83652