[gn build] allow setting clang_base_path to a source-absolute path

With this, you can set `clang_base_path = "//out/gn1"` in `out/gn2/args.gn` and
the build in out/gn2 will use clang and lld from out/gn1.

Setting `clang_base_path` to an absolute path (with e.g.
`clang_base_path = getenv("HOME") + "/src/..."`) should behave as before.

Differential Revision: https://reviews.llvm.org/D97989