To help with the transition to type-graph based CodeGen, we need to
reduce dependencies on legacy CodeGen from other parts of OI.
Pieces of CodeGen referenced by other files, but which only return
drgn data have been moved into a new "drgn_utils" target, allowing
us to remove CodeGen dependencies for these functions.
The new "symbol_service" build target can be used in the future by
other files only needing access to symbol information and not full
CodeGen.
Add CODE_COVERAGE option to enable coverage instrumentation in builds.
Add new CI job to upload coverage reports to CodeCov.io.
Reports can be found by navigating the codecov.io UI, or by going to
the URL printed at the end of the "Code Coverage" CI job. More CodeCov
integration could be set up later once we have coverage reporting on the
main branch.
