[llvm-dwarfdump][LineCov 1/3] Add variable coverage metrics (#176725)

Patch 1 of 3 to add to llvm-dwarfdump the ability to measure DWARF
coverage of local variables in terms of source lines, as discussed in
[this
RFC](https://discourse.llvm.org/t/rfc-debug-info-coverage-tool-v2/83266).

This patch adds the basic variable coverage implementation. By default,
inlined instances are shown separately (displaying the full inlining
chain). Alternatively, a combined view that averages across all inlined
instances can be returned using `--combine-instances`.

In this patch, we simply print a count of source lines over which each
variable is covered. Later patches in the series will add the comparison
against a baseline.

Example output:
```
$ llvm-dwarfdump --show-variable-coverage somefile
Variable coverage statistics:
Function InlChain                   Variable Decl                  LinesCovered
foo                                 bar      path/to/somefile.h:54 3
foo      path/to/someotherfile.c:32 bar      path/to/somefile.h:54 2
foo                                 baz      main.c:76             9
```

```
$ llvm-dwarfdump --show-variable-coverage somefile --combine-instances
Variable coverage statistics:
Function InstanceCount Variable Decl                  LinesCovered
foo      2             bar      path/to/somefile.h:54 2.5
foo      1             baz      main.c:76             9
```

This reapplies #169646, fixing some ambiguous overloads that caused
several bots to fail.