Print section start labels when first switching to the section.

Print section start labels when first switching to the section.

This is less brittle and avoids polluting the start of the file with every
debug section.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231898 91177308-0d34-0410-b5e6-96231b3b80d8