Avoid an infinite cycle with private linkage and -f{data|function}-sections.

When outputting an object we check its section to find its name, but when
looking for the section with -ffunction-section we look for the symbol name.

Break the loop by requesting a name with the private prefix when constructing
the section name. This matches the behavior before r201608.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201622 91177308-0d34-0410-b5e6-96231b3b80d8