[libc++] Add a .clang-tidy file to libcxx/src

This disables `readability-identifier-naming` for the source files,
since names don't have to by _Uglified in the source files. We currently
don't enforce clang-tidy in the source files, so this is only useful to
avoid a bunch of warnings when using an editor that shows the results of
clang-tidy.

Author: philnik777

libcxx/src/.clang-tidy

@@ -0,0 +1,4 @@
+InheritParentConfig: true
+
+Checks: >
+  -readability-identifier-naming