LLVMFlang: Fix MSVC ABI debug information format options

In commit 26bf32cdc6 (LLVMFlang: Add support for targeting MSVC ABI on
Windows, 2023-09-28, v3.28.0-rc1~10^2) we incorrectly recorded `-g` as
supporting the `ProgramDatabase` format, but it is actually `Embedded`,
matching Clang.

In order to support easy integration with C and C++ projects that use
the `.pdb` debug formats, pretend LLVMFlang supports them and just don't
actually emit any debug information.

Issue: #24840