[dsymutil] Emit label at the begin of a CU

[dsymutil] Emit label at the begin of a CU

When emitting a CU, store the MCSymbol pointing to the beginning of the
CU. We'll need this information later when emitting the .debug_names
section (DWARF5 accelerator table).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@336433 91177308-0d34-0410-b5e6-96231b3b80d8