MCAsmParser: change representation of MCAsmMacroParameter
MCAsmParser: change representation of MCAsmMacroParameter

Rather than using std::pair, create a structure to represent the type.  This is
a preliminary refactoring to enable required parameter handling.  Additional
state is needed to indicate required parameters.  This has a minor side effect
of improving readability by providing more accurate names compared to first and
second.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@201629 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,chubbymaggie/asap,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,chubbymaggie/asap,chubbymaggie/asap,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,dslab-epfl/asap,dslab-epfl/asap,dslab-epfl/asap,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,chubbymaggie/asap,dslab-epfl/asap,llvm-mirror/llvm,chubbymaggie/asap,llvm-mirror/llvm,dslab-epfl/asap,llvm-mirror/llvm,apple/swift-llvm,chubbymaggie/asap,apple/swift-llvm,llvm-mirror/llvm,dslab-epfl/asap,apple/swift-llvm,apple/swift-llvm,dslab-epfl/asap,apple/swift-llvm