[Dlang] option -Db_lto=true is ignored

**Describe the bug**
Option -Db_lto=true (link time optimization) is ignored at least for for ldc2 compiler

**To Reproduce**
Try to run `$ meson setup ---Db_lto=true builddir` and then run `ninja -v`

**system parameters**
* Is this a [cross build](https://mesonbuild.com/Cross-compilation.html) or just a plain native build (for the same computer)?
Both are affected

* what operating system (e.g. MacOS Catalina, Windows 10, CentOS 8.0, Ubuntu 18.04, etc.)
Debian

* what Python version are you using e.g. 3.8.0
Python 3.8.2

* what `meson --version`
~master

* what `ninja --version` if it's a Ninja build
1.10.0

No source file or test is named. Reproduce the issue with `meson setup ---Db_lto=true builddir` and `ninja -v`, then trace how Meson handles the option for the ldc2 compiler. Done means the verbose build shows the expected link-time optimization settings for both native and cross builds.

Written by the indexing model from the issue text.