Emit section information for extern variables.

Emit section information for extern variables.

Currently, if _attribute_((section())) is used for extern variables,
section information is not emitted in generated IR when the variables are used.
This is expected since sections are not generated for external linkage objects.
However NiosII requires this information as it uses special GP-relative accesses
for any objects that use attribute section (.sdata). GCC keeps this attribute in
  middle-end.

This change emits the section information for all targets.

Patch By: Elizabeth Andrews

Differential Revision:https://reviews.llvm.org/D36487

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@314262 91177308-0d34-0410-b5e6-96231b3b80d8