[DWARF5][Debuginfo] Compilation unit type (DW_UT_skeleton) and root DIE...

[DWARF5][Debuginfo] Compilation unit type (DW_UT_skeleton) and root DIE (DW_TAG_compile_unit) do not match.

That patch fixes incompatible compilation unit type (DW_UT_skeleton) and root DIE (DW_TAG_compile_unit) error.

cat split-dwarf.cpp
int main()
{
  int a = 1;
  return 0;
}

clang++ -O -g -gsplit-dwarf -gdwarf-5 split-dwarf.cpp; llvm-dwarfdump --verify ./a.out | grep skeleton
error: Compilation unit type (DW_UT_skeleton) and root DIE (DW_TAG_compile_unit) do not match.

The fix is to change DW_TAG_compile_unit into DW_TAG_skeleton_unit when skeleton file is generated.

Differential Revision: https://reviews.llvm.org/D70880