gn: define and use clang_base_path

Replace uses of "//third_party/llvm-build/Release+Asserts"
with the configurable option clang_base_path.

TBR=thakis
BUG=

Review-Url: https://codereview.chromium.org/2088373002
Cr-Commit-Position: refs/heads/master@{#401551}
10 files changed