[lib/MC] - Set SHF_EXCLUDE flag for .dwo sections.

DWARF5 spec says about single file split case:

"The sections that do not require relocation, however, can be written
to the relocatable object (.o) file but ignored by the
the linker or they can be written to a separate DWARF object (.dwo) file
that need not be accessed by the linker."

Nice way to make linker to ignore them is to set SHF_EXCLUDE flag.
It seems to be not harmful to always set it for .dwo sections.
That is what this patch does.

Differential revision: https://reviews.llvm.org/D52303

llvm-svn: 342800