[NFC] ][CLANG] Fix static code analyzer concerns

Reported by Coverity:

1. Inside "ASTReader.cpp" file,  in clang::ASTReader::FindExternalLexicalDecls(clang::DeclContext const *, llvm::function_ref<bool (clang::Decl::Kind)>, llvm::SmallVectorImpl<clang::Decl *> &): Using the auto keyword without an & causes a copy.

auto_causes_copy: Using the auto keyword without an & causes the copy of an object of type pair.

2. Inside "ASTReader.cpp" file, in clang::ASTReader::ReadAST(llvm::StringRef, clang::serialization::ModuleKind, clang::SourceLocation, unsigned int, llvm::SmallVectorImpl<clang::ASTReader::ImportedSubmodule> *): Using the auto keyword without an & causes a copy.

auto_causes_copy: Using the auto keyword without an & causes the copy of an object of type DenseMapPair.

3. Inside "CGOpenMPRuntimeGPU.cpp" file, in clang::CodeGen::CGOpenMPRuntimeGPU::emitGenericVarsEpilog(clang::CodeGen::CodeGenFunction &, bool): Using the auto keyword without an & causes a copy.

auto_causes_copy: Using the auto keyword without an & causes the copy of an object of type pair.

4. Inside "ASTWriter.cpp" file, in clang::ASTWriter::WriteHeaderSearch(clang::HeaderSearch const &): Using the auto keyword without an & causes a copy.

auto_causes_copy: Using the auto keyword without an & causes the copy of an object of type UnresolvedHeaderDirective.

Reviewed By: tahonermann

Differential Revision: https://reviews.llvm.org/D149461