Internals: Make AstCAwait an AstNodeStmt (#6280) (#7078)

AstCAwait is only ever uses in statement position, so model it as a
statement. We should never ever have a coroutine that returns a value.
There is no need for it in SV, nor should we rely on it for internals.

Also reworks the fix for V3Life incorrectly constant propagating the
beforeTrig functions (#7072). The property that upsets V3Life is that
a function:
1. Is called from multiple static call sites (multiple AstCCall)
2. Reads model state directly (AstVarRef to non-locals/arguments)

Such function can only be created internally after scheduling (V3Task
throws an unsupported error on a non-inlined function that reads model
state), so added a flag to AstCFunc to mark the dangerous ones for
V3Life.

src/V3Life.cpp

@@ -398,6 +398,7 @@ class LifeVisitor final : public VNVisitor {
         if (!m_tracingCall && !nodep->entryPoint()) return;
         m_tracingCall = false;
         if (nodep->recursive()) setNoopt();
+        if (nodep->noLife()) setNoopt();
         if (nodep->dpiImportPrototype() && !nodep->dpiPure()) {
             m_sideEffect = true;  // If appears on assign RHS, don't ever delete the assignment
         }