[analyzer]Fix another occurrence of iterator invalidation (LocalTUDecls)

Follow up in r155693, r155680.

Prevents a hard to reproduce crash with the following stack trace:
3  libsystem_c.dylib 0x00007ff55a835050 _sigtramp + 18446744029881443184
4  clang             0x0000000106218e97 (anonymous
namespace)::AnalysisConsumer::HandleTranslationUnit(clang::ASTContext&)
+ 519
5  clang             0x0000000105cf3002 clang::ParseAST(clang::Sema&,
bool, bool) + 690
6  clang             0x00000001059a41d8
clang::ASTFrontendAction::ExecuteAction() + 312
7  clang             0x00000001059a3df7 clang::FrontendAction::Execute()
+ 231
8  clang             0x00000001059b0ecc
clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) + 860
9  clang             0x000000010595e451
clang::ExecuteCompilerInvocation(clang::CompilerInstance*) + 961
10 clang             0x0000000105947f29 cc1_main(char const**, char
const**, char const*, void*) + 969
11 clang             0x0000000105958259 main + 473
12 clang             0x0000000105947b34 start + 52

llvm-svn: 157721