[ELF] - Add testcase documenting flags assigned when empty synthetic section is removed.

This responds to PR36475,

r325763 led to unexprected layout change, though
new behavior seems to be more correct.

Previously we could have following script:

.foo : { *(.foo) } 
.bar : { *(.synthetic_empty) BYTE(0x11) }}
where synthetic_empty is a synthetic section which is empty and
hence removed by linker.

Before r325763 .bar would receive section flags from .synthetic_empty,
but after this revision it receives flags the same as .foo section has.

It is the same as if there would not be any synthetic_empty section in a script,
so looks reasonable and consistent behavior:
.foo : { *(.foo) } 
.bar : { BYTE(0x11) }}

Patch adds testcase to document it.

Differential revision: https://reviews.llvm.org/D43632

llvm-svn: 325873