BitcodeWriter: Rename some functions for consistency, NFC

To match writeMetadataRecords, writeNamedMetadata and
writeMetadataStrings, change:

    WriteModuleMetadata        => writeModuleMetadata
    WriteFunctionLocalMetadata => writeFunctionMetadata
    Write##CLASS               => write##CLASS

The only major change is "FunctionLocal" => "Function".  The point is to
be less specific, in preparation for emitting normal metadata records
inside function metadata blocks (currently we only emit
`LocalAsMetadata` there).

llvm-svn: 264556