LLVM coverage does not process headers which are included via a search path

### Description of the bug:

When using `--experimental_use_llvm_covmap` and `--experimental_generate_llvm_lcov`, header files are only processed if they are included via a relative path (`#include "include/foo.h"`) rather than a search path (`includes = ["include"]` followed by `#include ` or `#include "foo.h"`).

### Which category does this issue belong to?

C++ Rules

### What's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

A minimal reproducer can be found at https://github.com/quuuuux/bazel_clang_coverage_reproducer. Flipping https://github.com/quuuuux/bazel_clang_coverage_reproducer/blob/master/test.cc#L3 from `1` to `0` demonstrates that the issue goes away if the header is included via a relative path.

### Which operating system are you running Bazel on?

Ubuntu 22.04 LTS

### What is the output of `bazel info release`?

release 6.2.1

### If `bazel info release` returns `development version` or `(@non-git)`, tell us how you built Bazel.

_No response_

### What's the output of `git remote get-url origin; git rev-parse HEAD` ?

_No response_

### Is this a regression? If yes, please try to identify the Bazel commit where the bug was introduced.

_No response_

### Have you found anything relevant by searching the web?

_No response_

### Any other information, logs, or outputs that you want to share?

Via search path:
```
cat bazel-out/_coverage/_coverage_report.dat
SF:lib.cc
...
end_of_record
SF:works.h
...
end_of_record
```

Via relative path:
```
cat bazel-out/_coverage/_coverage_report.dat
SF:include/doesntwork.h
...
end_of_record
SF:lib.cc
...
end_of_record
SF:works.h
...
end_of_record
```

Start with the minimal reproducer at quuuuux/bazel_clang_coverage_reproducer and compare test.cc line 3 with the two generated coverage reports. Trace the LLVM coverage handling for headers included through a search path versus a relative path; done means the search-path header appears in bazel-out/_coverage/_coverage_report.dat alongside the other files.

Written by the indexing model from the issue text.