Give Compiler a back reference to the driver.

The compiler driver is a single object delegating work to the compiler, rather
than passing it through to every Compiler call make it a member of Compiler so
that it maybe queried. This simplifies the Compiler API and makes the
relationship to CompilerDriver more explicit.
Remove reference arguments that contravene code style.

Change-Id: Iba47f2e3cbda679a7ec7588f26188d77643aa2c6