Fix several fixmes and clean up code by sinking *all* section

Fix several fixmes and clean up code by sinking *all* section
creation activity into the target-specific subclasses of TLOF.
Before this, globals with explicit sections could be created by
the base class.

1. make getOrCreateSection protected, add a new getExplicitSectionGlobal
   pure virtual method to assign sections to globals with a specified
   section.
2. eliminate getSpecialCasedSectionGlobals, which is now PIC specific.
3. eliminate the getKindForNamedSection virtual method, which is
   now just a static method for ELF.
4. Add implementions of getExplicitSectionGlobal for ELF/PECOFF/Darwin/PIC16.
   They are now all detangled and understandable, woo! :)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@78319 91177308-0d34-0410-b5e6-96231b3b80d8