Fix 26144; exception propagation for non-raising virtual methods (#26145)

ref #26144

The C backend must not use `sfNeverRaises` to remove exception checks
from
virtual method calls. The flag describes only the selected base method
body,
while a vtable override may raise a catchable exception.

This change makes `canRaiseDisp` conservative for `skMethod` symbols and
adds a
regression test covering an exception raised by a child method invoked
through a
base reference.

(cherry picked from commit 8cb406cd7a)

tests/method/tmethod_virtual_raise.nim

@@ -0,0 +1,20 @@
+discard """
+  output: '''caught'''
+"""
+
+type
+  Base = ref object of RootObj
+  Child = ref object of Base
+
+method run(value: Base): string {.base.} =
+  result = "base"
+
+method run(value: Child): string =
+  raise newException(ValueError, "child")
+
+let value: Base = Child()
+try:
+  discard value.run()
+  quit "virtual method did not raise"
+except ValueError:
+  echo "caught"