Upgrade LLVM

[alexcrichton]

This upgrade brings commit by @eddyb to help optimizations of virtual calls in
a few places (llvm-mirror/llvm@6d2bd95) as well as a
commit by @c-a to greatly improve the runtime of the optimization passes
(rust-lang/llvm#3).

Nice work to these guys!

[brson]

r+

[brson]

This doesn't touch the clean trigger.

[c-a]

Do the builders set LLVM_FORCE_USE_OLD_TOOLCHAIN? If so, this was probably broken by rust-lang/llvm@55e6c61. It renamed the option to LLVM_FORCE_USE_OLD_HOST_TOOLCHAIN but didn't update the code that tests for LLVM_FORCE_USE_OLD_TOOLCHAIN.

[c-a]

Sent a mail to llvm-commits about the option change: http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20140127/202996.html

[c-a]

Heh, on further inspection we use autoconf and not CMake to build llvm. So since rust-lang/llvm@7ba06ed we either need to update the compiler versions on the failing buildbots or pass --disable-compiler-version-checks to configure.

[alexcrichton]

Pushing this through try, it looks like we can build on the bots with --disable-compiler-version-checks, so I'm turning that on for now.

We'll probably need to look into upgrading the bots in the near future, however.

[alexcrichton]

Turns out LLVM changed how the dllexport "linkage" was set on global values, who knew!