[IR] Optimize memory usage of Metadata on MSVC
[IR] Optimize memory usage of Metadata on MSVC

An unsigned 2 bit bitfield takes 4 bytes in MSVC. Instead of a bitfield,
just use an unsigned char. We can go back to a bitfield when someone
implements the TODO of exposing and reusing the remaining 6 bits.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@266256 91177308-0d34-0410-b5e6-96231b3b80d8

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