[analyzer] Suppress optin.cplusplus.VirtualCall warnings in system headers (#184183)
Fixes #184178
The optin.cplusplus.VirtualCall checker reports warnings for virtual
method calls during construction/destruction even when the call site is
in a system header (included via -isystem). Users cannot fix such code
and must resort to NOLINT suppressions.
Add a system header check in checkPreCall before emitting the report,
consistent with how other checkers (e.g. MallocChecker) handle this.