Some language servers (vscode) seem to have started to rely on the
compiler being an absolute path recently. This updates the compdb
generator to match this expectation.
Bug: https://github.com/Microsoft/vscode-cpptools/issues/3112
Test: m nothing && m SOONG_GEN_COMPDB=1 SOONG_LINK_COMPDB_TO=$ANDROID_BUILD_TOP nothing
Test: Open vscode
Change-Id: Id146d69c2b808b23896132bbd4bc38ed5c7fd9c7