Development: add a script that caches clang-tidy results. (openscad#5304)

A self-contained c++ program that runs clang-tidy in parallel
on all files that changed since last invocation. It also takes
changed headers into acccount and re-processes all files that
depend on these.

Fully self-contaiend, so it can be invoked as a 'script' as
it compiles itself; a simple

    scripts/run-clang-tidy-cached.cc

works (but also invoking with 'sh -c' if you wish).

The results are cached in ~/.cache/clang-tidy out of tree, so
different checkouts or switching between branches
will also only need to process the different files.

The output are two files

  * OpenSCAD_clang-tidy.out : contains the detailed 'compiler warning'-like
    output (I usually use `cat OpenSCAD_clang-tidy.out` as 'compile command'
    in my editor to be able to step through it.
  * OpenSCAD_clang-tidy.summary : A summary with all finding counts, grouped
    by type (see openscad#5300 for an example).

A copy from run-clang-tidy-cached.cc from https://github.com/hzeller/dev-tools
with configuration for openscad; so the only difference from that upstream
script is the config-section:

```c++
static constexpr ConfigValues kConfig = {
  .cache_prefix = "OpenSCAD_",
  .start_dir = "src/",
  .file_exclude_re = "src/ext/",
  .revisit_brokenfiles_if_build_config_newer = false,  // not clean yet.
};
```

Fixes openscad#5300

Author: hzeller

.gitignore

@@ -95,3 +95,4 @@ tests/data/scad/misc/use-tests.scad
 tests/data/svg/viewbox/viewbox_*.svg
 tests/regression/pdfexporttest/centered.pdf
 tests/regression/pdfexporttest/simple-pdf.pdf
+scripts/run-clang-tidy-cached