[CMake] Stop re-linking

[rintaro]

What's in this pull request?

CC: @dabrahams
https://lists.swift.org/pipermail/swift-dev/Week-of-Mon-20160404/001621.html

${BUILD_DIR}/lib/Basic/SwiftRevision.inc used to be re-generated every time
git revision "hash" has changed. I feel, that is superfluous while developing.

With this change, {LLVM,Clang,Swift}Revision.inc generation is done
at configure time, instead of build time. That means:

• Fresh build: build everything normally
• Rebuild with --reconfigure: re-link and refresh --version output
• Rebuild without --reconfigure: no re-link and remain old --version output

AFAIK, every CI build has --reconfigure or perform scratch build.
So, this change should not affect CI bot behavior.

I confirmed:

$ utils/build-script -R --reconfigure  # fresh build
[.. build everything ..]
$ git commit --amend  # just change the hash
$ utils/build-script -R --reconfigure
[.. rebuild Version.cpp.o and re-linking ..]
$ vim stdlib/public/core/Algorithm.swift   # add one empty line
$ utils/build-script -R
[.. no re-linking .. build stdlib ..]
$ git commit -a  # change hash
$ utils/build-script -R
[.. rebuild nothing ..]
$ utils/build-script -R --reconfigure
[.. rebuild Version.cpp.o and re-linking ..]
$ utils/build-script -R --reconfigure
[.. rebuild nothing ..]

---

Before merging this pull request to apple/swift repository:

• Test pull request on Swift continuous integration.

[jrose-apple]

I'm concerned about this; it means that in a non-reconfiguring build, the revision hashes are lying to you.

There are also correctness implications; see my previous attempt at making it an option. #2105 (comment)

[rintaro]

Oh, I didn't know #2105. Thank you, closing.

[jrose-apple]

I would definitely like to make it an option, just not sure how to do so.