[CMake] Revert recent changes.

[gottesmm]

[CMake] Revert recent changes.

These changes caused a number of issues:

1. No debug info is emitted when a release-debug info compiler is built.
2. OS X deployment target specification is broken.
3. Swift options were broken without any attempt any recreating that
   functionality. The specific option in question is --force-optimized-typechecker.

Such refactorings should be done in a fashion that does not break existing
users and use cases.

This reverts commit e6ce2ff.
This reverts commit e8645f3.
This reverts commit 89b038e.
This reverts commit 497cac6.
This reverts commit 953ad09.
This reverts commit e096d1c.

rdar://30549345

[gottesmm]

@llvm-beanz I found these issues this evening. I am reverting since issues 1/3 are holding back engineering work. Feel free to recommit once these issues are fixed.

[gottesmm]

@swift-ci Please clean smoke test and merge

[jrose-apple]

Looks like it also broke the LTO build. @llvm-beanz, I can send you the link to the LTO bot if you want to see how it's configured.

[llvm-beanz]

The broken debug info is caused by an issue I discussed with @gottesmm back in October. Swift's build-script handles the CMAKE_${LANG}_FLAGS_${BUILD_TYPE} in non-standard and incorrect ways. I suspect the LTO issue is the same.

I'm re-applying the Export file changes that got reverted here as collateral in PR7529. These should have no impact on the swift build, but without them LLDB/upstream-with-swift no longer builds.

I may have time to fix build-script, but the refactoring changes are less important.