[LangRef] Clarify undefined behavior for function attributes.
[LangRef] Clarify undefined behavior for function attributes.

Violating the invariants specified by attributes is undefined behavior.
Maybe we could use poison instead for some of the parameter attributes,
but I don't think it's worthwhile.

Differential Revision: https://reviews.llvm.org/D49041



git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@337947 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm