When building SwitchStmts in Sema, record whether all the enum values of a switch...

When building SwitchStmts in Sema, record whether all the enum values of a switch(enum) where
covered by individual case statements.  Flow-based analyses may wish to consult this information,
and recording this in the AST allows us to obviate reconstructing this information later when
we build the CFG.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@113447 91177308-0d34-0410-b5e6-96231b3b80d8