[CodeView] Allow empty types in member functions

Summary:
`CodeViewDebug::lowerTypeMemberFunction` used to default to a `Void`
return type if the function's type array was empty. After D54667, it
started blindly indexing the 0th item for the return type, which fails
in `getOperand` for empty arrays if assertions are enabled.

This patch restores the `Void` return type for empty type arrays, and
adds a test generated by Rust in line-only debuginfo mode.

Reviewers: zturner, rnk

Reviewed By: rnk

Subscribers: hiraditya, JDevlieghere, llvm-commits

Differential Revision: https://reviews.llvm.org/D57070

llvm-svn: 351910