CompilationUnit only supports contiguous

### Description

1. Observation (high level) [buf_test.tar.gz](https://github.com/angr/cle/files/14831571/buf_test.tar.gz)
I tried to use angr's one feature to resolve the source level variable. I provided a source code as an example, a binary I compiled using Makefile, and the corresponding Python script to trace the value of a local variable. I believe you can replay this directly. Let me know if the script does not work.

2. Reasoning
From DAWRF 5 [manual](https://dwarfstd.org/doc/DWARF5.pdf) (page 61):
```
A full or partial compilation unit entry may have the following attributes:
1. Either a DW_AT_low_pc and DW_AT_high_pc pair of attributes or a
DW_AT_ranges attribute whose values encode the contiguous or
non-contiguous address ranges, respectively, of the machine instructions
generated for the compilation unit (see Section 2.17 on page 51).
```

There is no guarantee a compilation unit is contiguous, however, the class [CompilationUnit](https://github.com/angr/cle/blob/master/cle/backends/elf/compilation_unit.py#L10) requires the DWARF attributes including `low_pc` and `high_pc`. This implementation does not work in general. In my binary example, there is no compilation unit object for `main.c`. That is why no debug information after loading DWARF.

### Alternatives

The binary I compiled is using `-O2`. The compilation unit could be contiguous address space if `-O0` specifies. So I don't think this is a bug. This will be a new feature in angr cle if the CompilationUnit supports non-contiguous address ranges. Hopes you can add this feature on.

### Additional context

You can check the debug info I exported:
```
<0><314>: Abbrev Number: 16 (DW_TAG_compile_unit)
<315> DW_AT_producer : (indirect string, offset: 0xa): GNU C17 11.4.0 -mtune=generic -march=x86-64 -g -O2 -fasynchronous-unwind-tables -fstack-protector-strong -fstack-clash-protection -fcf-protection
<319> DW_AT_language : 29 (C11)
<31a> DW_AT_name : (indirect line string, offset: 0xb3): main.c
<31e> DW_AT_comp_dir : (indirect line string, offset: 0x0): /home/yusen/angr/buf_test
<322> DW_AT_ranges : 0xc
<326> DW_AT_low_pc : 0x0
<32e> DW_AT_stmt_list : 0x136
```
This compile unit has `DW_AT_ranges`.