Create unique, but identically-named ELF sections for explicitly-sectioned...

Create unique, but identically-named ELF sections for explicitly-sectioned functions and globals when using -function-sections and -data-sections.

This allows functions and globals to to be reordered later in the linking phase
(using the -symbol-ordering-file) even though reordering will be limited to
the scope of the explicit section.

Patch by Rahman Lavaee!

Differential Revision: https://reviews.llvm.org/D65478

llvm-svn: 367501