[Code Coverage] Add a tool to check test coverage of a patch

This script create a patch from the HEAD commit, extract modified or added source files, test case files
and source code lines, add coverage instrumentation for the affected source files, runs Lit tests,
and records which test cases cause each counter to be executed. Then report the number of test
cases executing the counter and the number of test cases executing the counter that are also changed
in some way by the patch. Thus providing developer the information of inadequately tested source lines.

Author: xgupta

llvm/utils/code-coverage/README.md

@@ -0,0 +1,59 @@
+# Patch based Code Coverage git integration
+
+git-check-coverage is a Python script designed to facilitate code coverage integration with Git. This tool allows developers to track code coverage for their commits, providing insights into which parts of the codebase are tested and which are not. One of the Future plan is to also integrate into github ci and provide code coverage data for each pull request to help reviewer in reviewing the changes.
+
+### Requirements
+
+    Python 3.x
+    Git
+    LLVM build tools (CMake, Ninja)
+    unidiff library (install via pip)
+    A recent version of clang/clang++ to build intrumented build directory
+    LLVM tools (llvm-cov, llvm-profdata, llvm-lit)
+
+### Setup & Usage
+
+```shell
+$ cd llvm-project
+$ export PATH=/home/user/local-llvm/bin:$PATH
+$ mkdir build
+$ cmake -G Ninja -Bbuild -Sllvm -DCMAKE_BUILD_TYPE=Release -DLLVM_USE_LINKER=lld -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DLLVM_ENABLE_ASSERTIONS=ON
+$ ninja -C build
+$ cp llvm/utils/git-check-coverage .
+$ chmod +x git-check-coverage
+$ git check-coverage -b build bin/opt llvm/test
+```
+
+Parameters
+
+    -b or --build-dir: Specify the build directory (default is build).
+    <binary_name>: Provide the name of the binary to analyze.
+    <test_suite_path>: Path to the test suite that will be executed.
+
+Optional Arguments
+
+    -n or --num-commits: Specify the number of recent commits to analyze.
+    -i or --instrumented-build-dir: Provide a separate directory for instrumented builds.
+
+> Note : clang and clang++ we are using is of recent version and install in path /home/user/local-llvm/bin.
+
+### Functions Overview
+
+The script contains several key functions:
+
+```
+extract_source_files_from_patch(patch_path): Extracts modified and test files from a patch.
+write_source_file_allowlist(source_files, allowlist_path): Writes an allowlist for source files.
+build_llvm(build_dir): Configures and builds LLVM in the specified build directory.
+run_modified_lit_tests(llvm_lit_path, patch_path, tests): Runs modified lit tests identified from the patch file.
+process_coverage_data(cpp_files, build_dir, binary): Converts profraw coverage data to profdata format and generates human-readable reports.
+read_coverage_file(coverage_file): Reads a coverage file to return covered and uncovered line numbers.
+print_common_uncovered_lines(cpp_file, uncovered_lines, modified_lines): Displays common uncovered lines for each modified source file.
+find_lit_tests(lit_path, test_paths): Identifies lit tests based on the provided paths.
+```
+
+### Known Issues
+
+1. Unit test
+2. LLDB tests(.py) coverage
+