Change test coverage generation to use llvm-cov instead of gcov.

Clang doesn't produce gcov compatible coverage files. This
causes lcov to break because it uses gcov by default. This
patch switches lcov to use llvm-cov as the gcov-tool.

Unfortunatly llvm-cov doesn't provide a gcov like interface by
default so it won't work with lcov. However `llvm-cov gcov` does.
For this reason we generate 'llvm-cov-wrapper' script that always
passes the gcov flag.

llvm-svn: 297553
diff --git a/libcxx/cmake/Modules/HandleCompilerRT.cmake b/libcxx/cmake/Modules/HandleCompilerRT.cmake
new file mode 100644
index 0000000..d7c4247
--- /dev/null
+++ b/libcxx/cmake/Modules/HandleCompilerRT.cmake
@@ -0,0 +1,43 @@
+function(find_compiler_rt_library name dest)
+  if (NOT DEFINED LIBCXX_COMPILE_FLAGS)
+    message(FATAL_ERROR "LIBCXX_COMPILE_FLAGS must be defined when using this function")
+  endif()
+  set(dest "" PARENT_SCOPE)
+  set(CLANG_COMMAND ${CMAKE_CXX_COMPILER} ${LIBCXX_COMPILE_FLAGS}
+      "--rtlib=compiler-rt" "--print-libgcc-file-name")
+  execute_process(
+      COMMAND ${CLANG_COMMAND}
+      RESULT_VARIABLE HAD_ERROR
+      OUTPUT_VARIABLE LIBRARY_FILE
+  )
+  string(STRIP "${LIBRARY_FILE}" LIBRARY_FILE)
+  string(REPLACE "builtins" "${name}" LIBRARY_FILE "${LIBRARY_FILE}")
+  if (NOT HAD_ERROR AND EXISTS "${LIBRARY_FILE}")
+    message(STATUS "Found compiler-rt library: ${LIBRARY_FILE}")
+    set(${dest} "${LIBRARY_FILE}" PARENT_SCOPE)
+  else()
+    message(STATUS "Failed to find compiler-rt library")
+  endif()
+endfunction()
+
+function(find_compiler_rt_dir dest)
+  if (NOT DEFINED LIBCXX_COMPILE_FLAGS)
+    message(FATAL_ERROR "LIBCXX_COMPILE_FLAGS must be defined when using this function")
+  endif()
+  set(dest "" PARENT_SCOPE)
+  set(CLANG_COMMAND ${CMAKE_CXX_COMPILER} ${LIBCXX_COMPILE_FLAGS}
+      "--rtlib=compiler-rt" "--print-libgcc-file-name")
+  execute_process(
+      COMMAND ${CLANG_COMMAND}
+      RESULT_VARIABLE HAD_ERROR
+      OUTPUT_VARIABLE LIBRARY_FILE
+  )
+  string(STRIP "${LIBRARY_FILE}" LIBRARY_FILE)
+  get_filename_component(LIBRARY_DIR "${LIBRARY_FILE}" DIRECTORY)
+  if (NOT HAD_ERROR AND EXISTS "${LIBRARY_DIR}")
+    message(STATUS "Found compiler-rt directory: ${LIBRARY_DIR}")
+    set(${dest} "${LIBRARY_DIR}" PARENT_SCOPE)
+  else()
+    message(STATUS "Failed to find compiler-rt directory")
+  endif()
+endfunction()