[llvm-strip] llvm-strip doesn't remove empty group sections

[chestnykh]

gcc -g3 produces ELF objects that contains group sections with .debug_ sections inside.
If such ELF object will be processed with llvm-strip --strip-debug then the output ELF will contain

group section [ 1] .group [foo_grp] contains 0 sections:.

Such objects cannot be processed for example by GNU binutils (file format not recognized).
The Full test case:

echo 'int main(){return 0;}' > test.c
gcc -g3 -c test.c -o test.o
llvm-strip --strip-debug test.o -o test.stripped.o
ld.bfd test.stripped.o -o test
test.stripped.o: file not recognized: file format not recognized

[llvmbot]

@llvm/issue-subscribers-tools-llvm-objcopy-strip

Author: Dmitriy Chestnykh (chestnykh)

`gcc -g3` produces ELF objects that contains group sections with `.debug_` sections inside. If such ELF object will be processed with `llvm-strip --strip-debug` then the output ELF will contain

group section [ 1] .group [foo_grp] contains 0 sections:.

Such objects cannot be processed for example by GNU binutils (file format not recognized).
The Full test case:

echo 'int main(){return 0;}' > test.c
gcc -g3 -c test.c -o test.o
llvm-strip --strip-debug test.o -o test.stripped.o
ld.bfd test.stripped.o -o test
test.stripped.o: file not recognized: file format not recognized