[llvm-cov][gcov] Support multi-files coverage in one basic block (#144504)

In the current gcov implementation, all lines within a basic block are
attributed to the source file of the block's containing function. This
is inaccurate when a block contains lines from other files (e.g., via
#include "foo.inc").

Commit
[406e81b](https://github.com/llvm/llvm-project/commit/406e81b79d26dae6838cc69d10a3e22635da09ef)
attempted to address this by filtering lines based on debug info types,
but this approach has two limitations:

* **Over-filtering**: Some valid lines belonging to the function are
incorrectly excluded.
* **Under-counting**: Lines not belonging to the function are filtered
out and omitted from coverage statistics.

**GCC Reference Behavior**
GCC's gcov implementation handles this case correctly.This change aligns
the LLVM behavior with GCC.

**Proposed Solution**
1. **GCNO Generation**:

* **Current**: Each block stores a single GCOVLines record (filename +
lines).

* **New**: Dynamically create new GCOVLines records whenever consecutive
lines in a block originate from different source files. Group subsequent
lines from the same file under one record.

2. **GCNO Parsing**:

* **Current**: Lines are directly attributed to the function's source
file.

* **New**: Introduce a GCOVLocation type to track filename/line mappings
within blocks. Statistics will reflect the actual source file for each
line.

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