llvm-mc/Mach-O: Don't put assembler temporary labels in the symbol table.

llvm-mc/Mach-O: Don't put assembler temporary labels in the symbol table.
 - I moved section creation back into AsmParser. I think policy decisions like
   this should be pushed higher, not lower, when possible (in addition the
   assembler has flags which change this behavior, for example).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@80162 91177308-0d34-0410-b5e6-96231b3b80d8