COFF: Stop requiring comdat sections to have an external leader to participate in ICF.

This requirement was added in r254578 to fix pr25686. However, it
appears to have originated from a misdiagnosis of the problem: link.exe
refused to merge the two sections because they are non-executable,
not because they have internal leaders. If I set up a similar scenario
with functions instead of globals I see that link.exe merges them.

Differential Revision: https://reviews.llvm.org/D40236

llvm-svn: 318682