NFC: silence some -Wrange-loop-analysis warnings.

[dan-zheng]

Silence warnings:

lib/AST/TypeWalker.cpp:42:21: warning: loop variable 'arg' of type 'const swift::Type' creates a copy from type 'const swift::Type' [-Wrange-loop-analysis]
    for (const auto arg : ty->getDirectGenericArgs())
                    ^
lib/AST/TypeWalker.cpp:42:10: note: use reference type 'const swift::Type &' to prevent copying
    for (const auto arg : ty->getDirectGenericArgs())
         ^~~~~~~~~~~~~~~~
                    &
1 warning generated.

lib/AST/ASTDumper.cpp:3527:23: warning: loop variable 'arg' of type 'const swift::Type' creates a copy from type 'const swift::Type' [-Wrange-loop-analysis]
      for (const auto arg : T->getDirectGenericArgs())
                      ^
lib/AST/ASTDumper.cpp:3527:12: note: use reference type 'const swift::Type &' to prevent copying
      for (const auto arg : T->getDirectGenericArgs())
           ^~~~~~~~~~~~~~~~
                      &
1 warning generated.

[dan-zheng]

@swift-ci Please smoke test

[dan-zheng]

I'll merge this now since I don't expect it to be controversial.