Update llvm to trunk r256633.

Merge llvm trunk r238337 from ^/vendor/llvm/dist, resolve conflicts, and
preserve our customizations, where necessary.

Merge llvm 3.5.0 release from ^/vendor/llvm/dist, resolve conflicts, and
preserve our customizations, where necessary.

Upgrade our copy of llvm/clang to trunk r154661, in preparation of the
upcoming 3.1 release (expected in a few weeks). Preliminary release
notes can be found at: <http://llvm.org/docs/ReleaseNotes.h

Upgrade our copy of llvm/clang to trunk r154661, in preparation of the
upcoming 3.1 release (expected in a few weeks). Preliminary release
notes can be found at: <http://llvm.org/docs/ReleaseNotes.html>

MFC after: 2 weeks

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Upgrade our copy of llvm/clang to r126079, from upstream's trunk.

This contains many improvements, primarily better C++ support, an
integrated assembler for x86 and support for -pg.