[analyzer] Explicit cast on customized offsetof should not be ignored when evaluating as const

If ignored, the subexpr is a UnaryOperator (&) which cannot be evaluated
(assertion failed).

  #define offsetof(type,memb) ((unsigned long)&((type*)0)->memb)

Patch By danix800!

Differential Revision: https://reviews.llvm.org/D144780

(cherry picked from commit 53f7542)

Author: danix800

clang/lib/StaticAnalyzer/Core/BugReporter.cpp

@@ -766,7 +766,7 @@ PathDiagnosticPieceRef PathDiagnosticBuilder::generateDiagForSwitchOP(
     case Stmt::CaseStmtClass: {
       os << "Control jumps to 'case ";
       const auto *Case = cast<CaseStmt>(S);
-      const Expr *LHS = Case->getLHS()->IgnoreParenCasts();
+      const Expr *LHS = Case->getLHS()->IgnoreParenImpCasts();
 
       // Determine if it is an enum.
       bool GetRawInt = true;

clang/test/Analysis/bitwise-ops-nocrash.c

@@ -0,0 +1,22 @@
+// RUN: %clang_analyze_cc1 -analyzer-checker=core -analyzer-output=text -triple x86_64-linux-gnu -Wno-shift-count-overflow -verify %s
+
+#define offsetof(type,memb) ((unsigned long)&((type*)0)->memb)
+
+typedef struct {
+  unsigned long guest_counter;
+  unsigned int guest_fpc;
+} S;
+
+// no crash
+int left_shift_overflow_no_crash(unsigned int i) {
+  unsigned shift = 323U; // expected-note{{'shift' initialized to 323}}
+  switch (i) { // expected-note{{Control jumps to 'case 8:'  at line 14}}
+  case offsetof(S, guest_fpc):
+    return 3 << shift; // expected-warning{{The result of the left shift is undefined due to shifting by '323', which is greater or equal to the width of type 'int'}}
+    // expected-note@-1{{The result of the left shift is undefined due to shifting by '323', which is greater or equal to the width of type 'int'}}
+  default:
+    break;
+  }
+
+  return 0;
+}