Put jump tables in unique sections on COFF.

Put jump tables in unique sections on COFF.

If a function is going in an unique section (because of -ffunction-sections
for example), putting a jump table in .rodata will keep .rodata alive and
that will keep alive any other function that also has a jump table.

Instead, put the jump table in a unique section that is associated with the
function.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231961 91177308-0d34-0410-b5e6-96231b3b80d8