[ObjectYAML] [DWARF] Abstract DWARF Initial Length values

In the DWARF 4 Spec section 7.2.2, data in many DWARF sections, and some DWARF structures start with "Initial Length Values", which are a 32-bit length, and an optional 64-bit length if the 32 bit value == UINT32_MAX.

This patch abstracts the Initial Length type in YAML, and extends its use to all the DWARF structures that are supported in the DWARFYAML code that have Initial Length values.

llvm-svn: 296911