[libc] update tidy rules to fix variable formatting

This commit changes the clang-tidy rules for LLVM-libc to follow the new
format. The next commit applies these rules to the codebase.

The rules are as follows:

CamelCase for classes
lower_case for variables
lower_case for functions
UPPER_CASE for constexpr variables

There are also some exceptions, but the most important one is that any
function or variable that starts with an underscore is exempt from the
formatting.

Reviewed By: sivachandra, lntue

Differential Revision: https://reviews.llvm.org/D114301

libc/CMakeLists.txt

@@ -55,6 +55,12 @@ if(LLVM_LIBC_ENABLE_LINTING)
   if("clang-tools-extra" IN_LIST LLVM_ENABLE_PROJECTS
              AND "clang" IN_LIST LLVM_ENABLE_PROJECTS)
     add_custom_target(lint-libc)
+    if(NOT EXISTS "${LIBC_BUILD_DIR}/.clang-tidy")
+      # add a no-op clang tidy file for the build directory so that the tidy 
+      # rules don't attempt to format the generated code.
+      file(WRITE ${LIBC_BUILD_DIR}/.clang-tidy "#InheritParentConfig: false
+      Checks: cppcoreguidelines-avoid-goto")
+    endif()
   else()
     message(FATAL_ERROR "
       'clang' and 'clang-tools-extra' are required in LLVM_ENABLE_PROJECTS to

libc/cmake/modules/LLVMLibCObjectRules.cmake

@@ -232,6 +232,7 @@ function(add_entrypoint_object target_name)
       # these.
       COMMAND $<TARGET_FILE:clang-tidy>
               "--extra-arg=-fno-caret-diagnostics" --quiet
+              "--export-fixes=${CMAKE_CURRENT_BINARY_DIR}/${target_name}.yaml"
               # Path to directory containing compile_commands.json
               -p ${PROJECT_BINARY_DIR}
               ${ADD_ENTRYPOINT_OBJ_SRCS}

libc/src/.clang-tidy

@@ -1,6 +1,24 @@
-Checks: '-*,llvmlibc-*'
+Checks: '-*,llvmlibc-*,readability-identifier-naming'
 HeaderFilterRegex: '.*'
 WarningsAsErrors: 'llvmlibc-*'
 CheckOptions:
   - key:             llvmlibc-restrict-system-libc-headers.Includes
     value:           '-*, linux/*, asm/*.h, asm-generic/*.h'
+  - key:             readability-identifier-naming.ClassCase
+    value:           CamelCase
+  - key:             readability-identifier-naming.StructCase
+    value:           aNy_CasE
+  - key:             readability-identifier-naming.MemberCase
+    value:           lower_case
+  - key:             readability-identifier-naming.VariableCase
+    value:           lower_case
+  - key:             readability-identifier-naming.VariableIgnoredRegexp
+    value:           "^_[A-Za-z0-9_]+$"
+  - key:             readability-identifier-naming.FunctionCase
+    value:           lower_case
+  - key:             readability-identifier-naming.FunctionIgnoredRegexp
+    value:           "^_[A-Za-z0-9_]+$"
+  - key:             readability-identifier-naming.ConstexprVariableCase
+    value:           UPPER_CASE
+  - key:             readability-identifier-naming.GetConfigPerFile
+    value:           true

libc/src/__support/CPP/.clang-tidy

@@ -0,0 +1,6 @@
+Checks: '-*,llvmlibc-*'
+HeaderFilterRegex: '.*'
+WarningsAsErrors: 'llvmlibc-*'
+CheckOptions:
+  - key:             llvmlibc-restrict-system-libc-headers.Includes
+    value:           '-*, linux/*, asm/*.h, asm-generic/*.h'