[AST][PCH][ASTImporter] Fix UB caused by uninited SwitchStmt member

The SwitchStmt::FirstCase member is not initialized when the AST is
built by the ASTStmtReader. See the below code of
ASTStmtReader::VisitSwitchStmt in the case where the for loop does not
have any iterations:
```
    // ... more code ...
    SwitchCase *PrevSC = nullptr;
    for (auto E = Record.size(); Record.getIdx() != E; ) {
      SwitchCase *SC = Record.getSwitchCaseWithID(Record.readInt());
      if (PrevSC)
        PrevSC->setNextSwitchCase(SC);
      else
        S->setSwitchCaseList(SC); // Sets FirstCase !!!

      PrevSC = SC;
    }
  } // return
```
Later, in ASTNodeImporter::VisitSwitchStmt,
we have a condition that depends on this uninited value:
```
  for (SwitchCase *SC = S->getSwitchCaseList(); SC != nullptr;
       SC = SC->getNextSwitchCase()) {
       // ... more code ...
  }

```
This is clearly an UB. This causes non-deterministic crashes when
ClangSA analyzes some code with CTU. See the below report by valgrind
(the whole valgrind output is attached):
```
==31019== Conditional jump or move depends on uninitialised value(s)
==31019==    at 0x12ED1983: clang::ASTNodeImporter::VisitSwitchStmt(clang::SwitchStmt*) (ASTImporter.cpp:6195)
==31019==    by 0x12F1D509: clang::StmtVisitorBase<std::add_pointer, clang::ASTNodeImporter, llvm::Expected<clang::Stmt*>>::Visit(clang::Stmt*) (StmtNodes.inc:591)
==31019==    by 0x12EE4FDF: clang::ASTImporter::Import(clang::Stmt*) (ASTImporter.cpp:8484)
==31019==    by 0x12F09498: llvm::Expected<clang::Stmt*> clang::ASTNodeImporter::import<clang::Stmt>(clang::Stmt*) (ASTImporter.cpp:164)
==31019==    by 0x12F3A1F5: llvm::Error clang::ASTNodeImporter::ImportArrayChecked<clang::Stmt**, clang::Stmt**>(clang::Stmt**, clang::Stmt**, clang::Stmt**) (ASTImporter.cpp:653)
==31019==    by 0x12F13152: llvm::Error clang::ASTNodeImporter::ImportContainerChecked<llvm::iterator_range<clang::Stmt**>, llvm::SmallVector<clang::Stmt*, 8u> >(llvm::iterator_range<clang::Stmt**> const&, llvm::SmallVector<clang::Stmt*, 8u>&) (ASTImporter.cpp:669)
==31019==    by 0x12ED099F: clang::ASTNodeImporter::VisitCompoundStmt(clang::CompoundStmt*) (ASTImporter.cpp:6077)
==31019==    by 0x12F1CC2D: clang::StmtVisitorBase<std::add_pointer, clang::ASTNodeImporter, llvm::Expected<clang::Stmt*>>::Visit(clang::Stmt*) (StmtNodes.inc:73)
==31019==    by 0x12EE4FDF: clang::ASTImporter::Import(clang::Stmt*) (ASTImporter.cpp:8484)
==31019==    by 0x12F09498: llvm::Expected<clang::Stmt*> clang::ASTNodeImporter::import<clang::Stmt>(clang::Stmt*) (ASTImporter.cpp:164)
==31019==    by 0x12F13275: clang::Stmt* clang::ASTNodeImporter::importChecked<clang::Stmt*>(llvm::Error&, clang::Stmt* const&) (ASTImporter.cpp:197)
==31019==    by 0x12ED0CE6: clang::ASTNodeImporter::VisitCaseStmt(clang::CaseStmt*) (ASTImporter.cpp:6098)
```

Differential Revision: https://reviews.llvm.org/D97849