Merge bitcoin/bitcoin#31933: doc: Add Clang/LLVM based coverage report generation

b96f1a696aa792068b5a4fa16e2d4a342e4f55b8 add clang/llvm based coverage report generation (Prabhat Verma)

Pull request description:

  Followed up from the [comment](https://github.com/bitcoin/bitcoin/issues/31927#issuecomment-2674522975) on the issue [#31927](https://github.com/bitcoin/bitcoin/issues/31927) , issues have been observed building coverage reports with `gcov` in MacOs and NixOs. This PR adds the steps to generate a coverage report based on the default llvm/clang tooling.

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Tree-SHA512: bc54f170e84bb76b3eba7285bd49f051c0b99b784d583a550d8e51511497bcc4df8964bbe3991777648d2f829809db8eabb0cbf0d25f9da5e49e1cfc62f6d8d0

doc/developer-notes.md

@@ -487,6 +487,8 @@ $ ./build/test/functional/test_runner.py --valgrind
 
 ### Compiling for test coverage
 
+#### Using LCOV
+
 LCOV can be used to generate a test coverage report based upon `ctest`
 execution. LCOV must be installed on your system (e.g. the `lcov` package
 on Debian/Ubuntu).
@@ -516,6 +518,63 @@ To enable test parallelism:
 cmake -DJOBS=$(nproc) -P build/Coverage.cmake
 ```
 
+#### Using LLVM/Clang toolchain
+
+The following generates a coverage report for unit tests and functional tests.
+
+Configure the build with the following flags:
+
+> Consider building with a clean state using `rm -rf build`
+
+```shell
+# MacOS may instead require `-DCMAKE_C_COMPILER="$(brew --prefix llvm)/bin/clang" -DCMAKE_CXX_COMPILER="$(brew --prefix llvm)/bin/clang++"`
+cmake -B build -DCMAKE_C_COMPILER="clang" \
+   -DCMAKE_CXX_COMPILER="clang++" \
+   -DAPPEND_CFLAGS="-fprofile-instr-generate -fcoverage-mapping" \
+   -DAPPEND_CXXFLAGS="-fprofile-instr-generate -fcoverage-mapping" \
+   -DAPPEND_LDFLAGS="-fprofile-instr-generate -fcoverage-mapping"
+cmake --build build # Use "-j N" here for N parallel jobs.
+```
+
+Generating the raw profile data based on `ctest` and functional tests execution:
+
+```shell
+# Create directory for raw profile data
+mkdir -p build/raw_profile_data
+
+# Run tests to generate profiles
+LLVM_PROFILE_FILE="$(pwd)/build/raw_profile_data/%m_%p.profraw" ctest --test-dir build # Use "-j N" here for N parallel jobs.
+LLVM_PROFILE_FILE="$(pwd)/build/raw_profile_data/%m_%p.profraw" build/test/functional/test_runner.py # Use "-j N" here for N parallel jobs
+
+# Merge all the raw profile data into a single file
+find build/raw_profile_data -name "*.profraw" | xargs llvm-profdata merge -o build/coverage.profdata
+```
+
+> **Note:** The "counter mismatch" warning can be safely ignored, though it can be resolved by updating to Clang 19.
+> The warning occurs due to version mismatches but doesn't affect the coverage report generation.
+
+Generating the coverage report:
+
+```shell
+llvm-cov show \
+    --object=build/bin/test_bitcoin \
+    --object=build/bin/bitcoind \
+    -Xdemangler=llvm-cxxfilt \
+    --instr-profile=build/coverage.profdata \
+    --ignore-filename-regex="src/crc32c/|src/leveldb/|src/minisketch/|src/secp256k1/|src/test/" \
+    --format=html \
+    --show-instantiation-summary \
+    --show-line-counts-or-regions \
+    --show-expansions \
+    --output-dir=build/coverage_report \
+    --project-title="Bitcoin Core Coverage Report"
+```
+
+> **Note:** The "functions have mismatched data" warning can be safely ignored, the coverage report will still be generated correctly despite this warning.
+> This warning occurs due to profdata mismatch created during the merge process for shared libraries.
+
+The generated coverage report can be accessed at `build/coverage_report/index.html`.
+
 ### Performance profiling with perf
 
 Profiling is a good way to get a precise idea of where time is being spent in