clover: make GCC 4.8 happy

clover: make GCC 4.8 happy

Without this GCC 4.8.x throws below error:

error: invalid initialization of non-const reference of type
'clover::llvm::compat::raw_ostream_to_emit_file {aka llvm::raw_svector_ostream&}'
from an rvalue of type '<brace-enclosed initializer list>'

v2: change commit title and add error message like Eric Engestrom requested

Signed-off-by: Dieter Nützel <Dieter@nuetzel-hh.de>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97019
[ Francisco Jerez: Trivial formatting fix. ]
Reviewed-by: Francisco Jerez <currojerez@riseup.net>