[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