Internals: Replace AstMTaskBody with AstCFunc(#6280) (#6628)

AstMTaskBody is somewhat redundant and is problematic for #6280. We used
to wrap all MTasks in a CFunc before emit anyway. Now we create that
CFunc when we create the ExecMTask in V3OrderParallel, and subsequently
use the CFunc to represent the contents of the MTask. Final output and
optimizations are the same, but internals are simplified to move
towards #6280.

No functional change.

src/V3LifePost.cpp

@@ -290,7 +290,7 @@ class LifePostDlyVisitor final : public VNVisitorConst {
             const ExecMTask* const mtaskp = mtaskVtx.as<ExecMTask>();
             VL_RESTORER(m_execMTaskp);
             m_execMTaskp = mtaskp;
-            iterateConst(mtaskp->bodyp());
+            trace(mtaskp->funcp());
         }
     }
     void visit(AstCFunc* nodep) override {