Add instructions and scripts on how to generate a coverage report

Enhancement

https://github.com/pingcap/tiflash/blob/739d1685ab787b0f926afe8d43e15fffc2aa2926/README.md#generate-llvm-coverage-report

Reference: https://clang.llvm.org/docs/SourceBasedCodeCoverage.html

A possible scripts

set -x

## Reference: https://clang.llvm.org/docs/SourceBasedCodeCoverage.html

PROF_DIR="/DATA/disk1/jaysonhuang/tiflash/cmake-build-debug-cov"
# Build the testing binary
cd ${PROF_DIR} && cmake .. -GNinja -DCMAKE_BUILD_TYPE=Debug -DTEST_LLVM_COVERAGE=ON && ninja -j 32 gtests_dbms && cd -

# Run the testing binary and generate prof raw data
#LLVM_PROFILE_FILE="${PROF_DIR}/gtest_dbms.profraw" ${PROF_DIR}/dbms/gtests_dbms --gtest_filter='*DeltaMergeStore*:*DMFile*:*Segment*' 2>&1 | tee ${PROF_DIR}/coverage_test.output.log
LLVM_PROFILE_FILE="${PROF_DIR}/gtest_dbms.profraw" ${PROF_DIR}/dbms/gtests_dbms 2>&1 | tee ${PROF_DIR}/coverage_test.output.log


# Collect the prof raw data and generate cov report
llvm-profdata merge -sparse ${PROF_DIR}/*.profraw -o ${PROF_DIR}/merged.profdata

TEST_BIN_PATH="${PROF_DIR}"
TEST_BINS="${TEST_BIN_PATH}/dbms/gtests_dbms"
                    
export LD_LIBRARY_PATH=.
llvm-cov export \
    ${TEST_BINS} \
    --format=lcov \
    --instr-profile ${PROF_DIR}/merged.profdata \
    --ignore-filename-regex "/usr/include/.*" \
    --ignore-filename-regex "/usr/local/.*" \
    --ignore-filename-regex "/usr/lib/.*" \
    --ignore-filename-regex ".*/contrib/.*" \
    --ignore-filename-regex ".*/dbms/src/Debug/.*" \
    --ignore-filename-regex ".*/dbms/src/Client/.*" \
    > ${PROF_DIR}/lcov.info

mkdir -p "${PROF_DIR}/report"
genhtml "${PROF_DIR}/lcov.info" -o "${PROF_DIR}/report/" --ignore-errors source

Start with the README.md coverage section linked in the issue and the referenced LLVM source-based coverage documentation. Turn the proposed build, test, llvm-profdata, llvm-cov, and genhtml commands into documented instructions and scripts that work for the project; done means a contributor can generate an HTML coverage report.