BitcodeWriter: Split out writeMetadataRecords, NFC
BitcodeWriter: Split out writeMetadataRecords, NFC

Besides being a nice cleanup, this is preparation for reusing the code
in function metadata blocks.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@264555 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm