[NFC] Get rid of -Wrange-loop-analysis warnings.

[varungandhi-apple]

Keep seeing annoying warnings like

[979/1334][ 73%][66.143s] Building CXX object lib/AST/CMakeFiles/swiftAST.dir/Decl.cpp.o
/Users/varun/foss-swift/swift/lib/AST/Decl.cpp:1831:21: warning: loop variable 'elt' of type 'const swift::TupleTypeReprElement' creates a copy from type 'const swift::TupleTypeReprElement' [-Wrange-loop-analysis]
    for (const auto elt : tuple->getElements()) {
                    ^
/Users/varun/foss-swift/swift/lib/AST/Decl.cpp:1831:10: note: use reference type 'const swift::TupleTypeReprElement &' to prevent copying
    for (const auto elt : tuple->getElements()) {
         ^~~~~~~~~~~~~~~~
                    &

Stop it.

[varungandhi-apple]

@swift-ci please test source compatibility

[varungandhi-apple]

@swift-ci please test