Replace the general `createLocalVariable()` with two more specific
functions: `createParameterVariable()` and `createAutoVariable()`, and
rewrite the documentation.
Besides cleaning up the API, this avoids exposing the fake DWARF tags
`DW_TAG_arg_variable` and `DW_TAG_auto_variable` to frontends, and is
preparation for removing them completely.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243764 91177308-0d34-0410-b5e6-96231b3b80d8