clike compiler args: ignore -Xclang it and whatever comes after it

[aleden]

I wanted to build QEMU under clang with a custom plugin (for the compiler itself). Via --extra-cflags, I shuttled the following into the CFLAGS:

-Xclang -load -Xclang /usr/local/lib/carbon-collect.so -Xclang -add-plugin -Xclang carbon-collect -Xclang -plugin-arg-carbon-collect -Xclang "$(pwd)" -Xclang -plugin-arg-carbon-collect -Xclang "$(pwd)"

This did not build, meson threw an error, because it bungled the arguments given to the compiler, see #11184 for more details.

I believe that when meson goes about figuring out '-Wl,--start-group' and '-Wl,--end-group', it assumes that /usr/local/lib/carbon-collect.so is a DSO in the build, when it is in fact a plugin to the compiler itself (which happens to be a shared library).

It seems to me that anything could come after an '-Xclang'. Do other compilers out there have an analogous command-line option? Does this need to be configurable?