[NFC][clang] Fix coverity static analyzer concerns about AUTO_CAUSES_COPY

Reported by Coverity:

AUTO_CAUSES_COPY
Unnecessary object copies can affect performance.

1. [NFC] Fix auto keyword use without an & causes the copy of an object of type SimpleRegistryEntry in clang::getAttributePluginInstances()

2. [NFC] Fix auto keyword use without an & causes the copy of an object of type tuple in CheckStmtInlineAttr<clang::NoInlineAttr, 2>(clang::Sema &, clang::Stmt const *, clang::Stmt const *, clang::AttributeCommonInfo const &)

3. [NFC] Fix auto keyword use without an & causes the copy of an object of type QualType in <unnamed>::SystemZTargetCodeGenInfo::isVectorTypeBased(clang::Type const *, bool)

4. [NFC] Fix auto keyword use without an & causes the copy of an object of type Policy in <unnamed>::RISCVIntrinsicManagerImpl::InitIntrinsicList()

5. [NFC] Fix auto keyword use without an & causes the copy of an object of type pair in checkUndefinedButUsed(clang::Sema &)

Reviewed By: tahonermann

Differential Revision: <https://reviews.llvm.org/D147543>