Add interceptor support for symbols

a revival of https://codereview.chromium.org/467013003

R=rossberg@chromium.org

BUG=

Review URL: https://codereview.chromium.org/760883002

Cr-Commit-Position: refs/heads/master@{#25537}
Add support for downloading clang

Still requires changes to gyp files to select the built-in clang as
compiler and make clang the default on the platforms we want it.

BUG=
R=dcarney@chromium.org,machenbach@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/745963005

Cr-Commit-Position: refs/heads/master@{#25538}