should have no state that is specific to particular globals in the
section.  In this case, it means the removal of the "isWeak" and
"ExplicitSection" bits.  MCSection uses the new form of SectionKind.

To handle isWeak, I introduced a new SectionInfo class, which is
SectionKind + isWeak, and it is used by the part of the code generator
that does classification of a specific global.

The ExplicitSection disappears.  It is moved onto MCSection as a new
"IsDirective" bit.  Since the Name of a section is either a section
or directive, it makes sense to keep this bit in MCSection.  Ultimately
the creator of MCSection should canonicalize (e.g.) .text to whatever
the actual section is.

llvm-svn: 77803