Move llvm:Error types

[etcwilde]

Older compilers don't have the copy-elision rule of newer C++ compilers, resulting in an error message when returning a non-copyable type. Move llvm::Error instead of copying it.

/home/ewilde/work/swift-project/swift/lib/Frontend/CachingUtils.cpp:93:16: error: call to deleted constructor of 'llvm::Error'
        return Err;
               ^~~
/home/ewilde/work/swift-project/llvm-project/llvm/include/llvm/Support/Error.h:184:3: note: 'Error' has been explicitly marked deleted here
  Error(const Error &Other) = delete;
  ^
/home/ewilde/work/swift-project/llvm-project/llvm/include/llvm/Support/Error.h:491:18: note: passing argument to parameter 'Err' here
  Expected(Error Err)
                 ^
/home/ewilde/work/swift-project/swift/lib/Frontend/CachingUtils.cpp:123:16: error: call to deleted constructor of 'llvm::Error'
        return Err;
               ^~~
/home/ewilde/work/swift-project/llvm-project/llvm/include/llvm/Support/Error.h:184:3: note: 'Error' has been explicitly marked deleted here
  Error(const Error &Other) = delete;
  ^
/home/ewilde/work/swift-project/llvm-project/llvm/include/llvm/Support/Error.h:491:18: note: passing argument to parameter 'Err' here
  Expected(Error Err)
                 ^

[etcwilde]

@swift-ci please smoke test

[cachemeifyoucan]

LGTM.