[LyX/master] Add move constructor and move assignment operator for FileName class

[Yuriy Skalko]

> Sorry, I’ve reverted the change for now locally. I can answer your questions 
> later…
> 
> Perhaps the compiler flags of the autotools build are of interest (but cmake 
> build crashes either):
> 
> Configuration
>   Host type:               x86_64-apple-darwin18.7.0
>   Special build flags:      build=release warnings callback-printing 
> use-hunspell use-aspell
>   Bundled libraries:        nod boost mythes
>   C++ Compiler:            c++ -stdlib=libc++ (11.0.0)
>   C++ Compiler flags:       -Wall -Wextra -fPIC -Os -std=c++17  
> -Wno-deprecated-register
>   C++ Compiler user flags:  -I/Users/Shared/LyX/utilities/include   -isysroot 
> /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk
>  -arch x86_64 -mmacosx-version-min=10.10 -std=c++17  -std=c++11
>   Linker flags:             -rdynamic
>   Linker user flags:        -L/Users/Shared/LyX/utilities/lib -isysroot 
> /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk
>  -arch x86_64 -mmacosx-version-min=10.10
>   Qt Frontend:
>       Qt version:          5.12.9
>   Packaging:               macosx
> 
> Are all these library and compiler standard choice switches compatible and 
> correct?
> 
> Stephan


Thanks for testing Stephan and Scott.
So seems like it is macos-only issue. I see nothing suspicious in your 
compiler flags (are you manually override default C++17 to C++11?).

The patch is already reverted in master branch.


Yuriy