Added '-Blinker' frontend driver option for cross-linker search path

[karwa]

What's in this pull request?

Added a -Blinker fronted driver option. This works like clang/gcc's "-B" flag (which allows specifying an alternate toolchain location).

Currently, you can cross-compile with swift (assuming you've got a copy of the standard library compiled for the target), like so:
swiftc -target armv7-unknown-linux-gnueabihf -sdk ${sysroot} -resource-dir ${sdk-location} main.swift

However, this only works for modules (swiftmodules), not for executables. The host swift generates a link command to clang++ with -fuse-ld=gold, as it should, but there is currently no way to tell it where to find the gold cross-linker. Normally, you would put this information behind the '-B' switch, so we provide the same kind of functionality here.

Adding -Blinker ${gcc-toolchain-path} to the above command allows swift to cross-compile executables.

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[jrose-apple]

Thanks for looking at this! That's not a very Swifty flag name though (we don't really do the prefix thing), and I'm leery of adding Linux-only options. How is this flag used, i.e. what is ${gcc-toolchain-path}?

[lattner]

I think we should actually design the proper cross-toolchain solution as a real feature. That said, I don't think it is reasonable to block all progress on getting ports going on tackling such a large design topic. In the meantime, how about we carve out a namespace for "experimental" command line options and put them under that? @jrose-apple what do you think about that?

[karwa]

${gcc-toolchain-path} is the directory where ld.gold is (or whichever linker); It's the directory and not the exact path, because of the way it gets forwarded to clang.

I liked the name blinker, it's strange yet appropriate; we have -I and -L and -use-ld (with the equals and everything), Xlinker and so on, so I went for familiarity. I'm all for renaming, though; sticking this feature behind the "-B" flag isn't obvious, even for clang/gcc.

As for sticking it in an 'experimental' scope; this flag is basically the counterpart to -use-ld and as long as we have that flag allowing you to specify a linker, it is necessary to have a flag letting you specify where said linker lives. If we hide it away somewhere, we should do the same to -use-ld. FYI, I needed this right now in order to cross-compile LLDB (repl_swift produces an executable), so there is a imminent need for it.

I would support renaming both options, -use-ld should become -linker-name and -Blinker should become -linker-searchpath

[jrose-apple]

Alternative: -use-ld actually takes a full path in recent Clangs, doesn't it? I'm not sure we've pulled that change into apple/swift-clang's "stable" branch yet, but that would be good enough to make progress, right?

[karwa]

Yes, I didn't know that, but you're right: http://reviews.llvm.org/rL262996

We haven't pulled that in, but it would be great if we did. I'd also want to rename -use-ld, because it is a bit legacy, but that's less important than having the functionality there.

[jrose-apple]

It's already in the swift-3.0-branch for apple/swift-clang, so feel free to cherry-pick it into stable as well (via pull request).

[karwa]

This is no longer needed, absolute paths in clang is now in stable after the latest pull