cmake: add native clang-tidy support for tests, with concatenated sources

[vszakats]

Tests are build in "unity"-style, by including sources into an umbrella
C files (similar to how CMake unity works). This does not play well with
clang-tidy, which seems to unconditionally ignore C sources included
like this. To fix it, curl's CMake implements a manual clang-tidy
support for tests, which compiles sources one-by-one, while also making
sure sources compile cleanly standalone (e.g. all sources need to
include first.h). The manual clang-tidy implementation is fragile, and
performance, in particular when targeting Windows, is abysmal.

This patch introduces an alternate solution, enabled by the
_CURL_TESTS_CONCAT=ON option. In this mode, umbrella sources include
the actual sources instead of #including them. Allowing to use CMake's
built-in clang-tidy support to compile them, with clang-tidy actually
checking the sources. Making the manual clang-tidy support unnecessary.
In the Windows CI job it results in a 4x performance improvement (4m →
1m), making it practical to run clang-tidy on tests on Windows, in CI.

The main downside is that clang-tidy doesn't understand the #line
directive. Meaning issues found show the wrong filename and line number
next to them. It's not impossible to locate errors this way, but also
not convenient.

Minor/potential downside is that the concatenated source needs to be
reassembled each time an original source is updated. This may result in
more copying on the disk when used in local development. The largest
source is 1.4MB, so probably not a show-stopper on most machines.

Another is the complexity of maintaining two methods in parallel, which
may be necessary till clang-tidy understands #line:
llvm/llvm-project#62405

This solution may in theory also enable adding clang-tidy support for
tests in autotools, though I haven't tried.

Targeted for curl CI for now, and used in a GHA/windows job. 100%
experimental, not recommended outside these.

---

https://github.com/curl/curl/pull/20667/files?w=1

• this mode uses CMake's built-in clang-tidy invocation, removing
  the fragile workaround trying to emulate it manually.
  (via curl_add_clang_tidy_test_target())
  This possibly makes it work in more cross-build scenarios
  (and possibly other corner-cases), though I haven't stress-tested
  it yet.
• this mode can be restricted to clang-tidy builds, though it
  plays well with normal complation.
  (minor downside: umbrealla file needs to be regenerated
  on source changes.)
• runtime: 2m11 (was: 5m30 GHA/windows: try enabling clang-tidy for tests #20631)
  Ref: https://github.com/curl/curl/actions/runs/22267951773/job/64417510535?pr=20667
  Still slow partly due to curlu and curltool libs. → down to 1m8 with unity force-enabled for curlu and curltool ref: https://github.com/curl/curl/actions/runs/22270239494/job/64432225756?pr=20667. 5x speedup. also 1m8 with alternate, better patch: https://github.com/curl/curl/actions/runs/22280029021/job/64449085738?pr=20667

---

• requires lib: use lib source directory as base include path #20623, merged and rebased.
• delete commit 20657. only necessary for local env.