[OPENMP] Initial codegen for 'parallel sections' directive.

[OPENMP] Initial codegen for 'parallel sections' directive.

Emits code for outlined 'parallel' directive with the implicitly inlined 'sections' directive:

...
call __kmpc_fork_call(..., outlined_function, ...);
...

define internal void outlined_function(...) {
    <code for implicit sections directive>;
}
Differential Revision: http://reviews.llvm.org/D8997

llvm-svn: 234849