Fix Clang tooling build on Linux.
Since the chrome clang tools are built outside the LLVM tree, direct
references to LLVM/Clang libraries don't work as expected on Linux.
This patch changes the Clang update script to create a shim directory
for triggering the Chrome tools build as part of the LLVM build. The
shim CMakeLists.txt just forwards to the real one in tools/clang. As
a bonus, this allows the CMake files in tools/clang to just use
CMAKE_SOURCE_DIR/CMAKE_BINARY_DIR instead of having to manually
specify the LLVM source/binary dirs. In addition, with this change,
it's possible to have incremental builds of tools, since the script
no longer blows away the output directory.
Review URL: https://codereview.chromium.org/
615083006
Cr-Commit-Position: refs/heads/master@{#298152}