[RFC] CMake: Set custom RelWithDebInfo compile flags.

Follow-up from #1366. Tested that it uses the correct flags with something like:

```
$ export PATH=$HOME/bin:$PATH
$ cat > $HOME/bin/cc
#!/bin/bash
echo "$*" >> $HOME/debugcc
/usr/bin/gcc $*
$ make CMAKE_BUILD_TYPE=NvimReleaseWithDebInfob
$ less $HOME/debugcc
`grep -r "ifdef DEBUG$" src` shows only a few places where DEBUG is used.. one of them would be removed in #1403. Updated the PR and removed this change. Assertions should work regardless, right? (as long as NDEBUG is not set)