Fix undefined behavior in DWARF emission

Caused by uninitialized load of llvm::DwarfDebug::PrevCU:
llvm::DwarfCompileUnit::addRange () at ../lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp:276
llvm::DwarfDebug::endFunctionImpl () at ../lib/CodeGen/AsmPrinter/DwarfDebug.cpp:1586
llvm::DebugHandlerBase::endFunction () at ../lib/CodeGen/AsmPrinter/DebugHandlerBase.cpp:319
llvm::AsmPrinter::EmitFunctionBody () at ../lib/CodeGen/AsmPrinter/AsmPrinter.cpp:1230
llvm::ARMAsmPrinter::runOnMachineFunction () at ../lib/Target/ARM/ARMAsmPrinter.cpp:161

Most of the DebugInfo tests under `LLVM_LIT_ARGS:STRING=-sv --vg` prior to this fix, and pass with the fix applied.

Reviewed By: aprantl, dblaikie

Differential Revision: https://reviews.llvm.org/D81631