Make all expressions derive from AstNodeExpr (#3721).

Apart from the representational changes below, this patch renames
AstNodeMath to AstNodeExpr, and AstCMath to AstCExpr.

Now every expression (i.e.: those AstNodes that represent a [possibly
void] value, with value being interpreted in a very general sense) has
AstNodeExpr as a super class. This necessitates the introduction of an
AstStmtExpr, which represents an expression in statement position, e.g :
'foo();' would be represented as AstStmtExpr(AstCCall(foo)). In exchange
we can get rid of isStatement() in AstNodeStmt, which now really always
represent a statement

Peak memory consumption and verilation speed are not measurably changed.

Partial step towards #3420

src/V3AstInlines.h

@@ -141,8 +141,8 @@ AstCStmt::AstCStmt(FileLine* fl, const string& textStmt)
     addExprsp(new AstText{fl, textStmt, true});
 }
 
-AstCMath::AstCMath(FileLine* fl, const string& textStmt, int setwidth, bool cleanOut)
-    : ASTGEN_SUPER_CMath(fl)
+AstCExpr::AstCExpr(FileLine* fl, const string& textStmt, int setwidth, bool cleanOut)
+    : ASTGEN_SUPER_CExpr(fl)
     , m_cleanOut{cleanOut}
     , m_pure{true} {
     addExprsp(new AstText{fl, textStmt, true});
@@ -180,4 +180,6 @@ AstVarXRef::AstVarXRef(FileLine* fl, AstVar* varp, const string& dotted, const V
     dtypeFrom(varp);
 }
 
+AstStmtExpr* AstNodeExpr::makeStmt() { return new AstStmtExpr{fileline(), this}; }
+
 #endif  // Guard