Drop the redundant semicolon from CAPTURE() call sites; remove -Wno-extra-semi-stmt

doctest_compatibility.h defines CAPTURE(x) as DOCTEST_CAPTURE(x); (with
a trailing semicolon baked into the macro), specifically so call sites
do not need to add one themselves; most of the ~267 call sites already
follow that convention. The remaining 64 call sites across 20 files
wrote "CAPTURE(x);" anyway, turning into a statement plus an empty
statement and triggering -Wextra-semi-stmt. Dropped the redundant
semicolon at each of those sites.

With item 6 having already made vendored Doctest a SYSTEM include, and
this the last known source of -Wextra-semi-stmt findings, removed the
flag from clang_flags.cmake entirely.

Verified with clang++ -Wextra-semi-stmt (plus the file's other CI
ignores) that all 20 touched files, plus a file with no CAPTURE() use
(unit-json_pointer.cpp), compile without any -Wextra-semi-stmt
diagnostic.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

#5725 item 8 (step 2 of 2)

tests/src/unit-comparison.cpp

@@ -838,7 +838,7 @@ TEST_CASE("equality of objects whose entries have no fixed order")
 
     for (const std::size_t depth : std::vector<std::size_t> {0, 200})
     {
-        CAPTURE(depth);
+        CAPTURE(depth)
 
         const unordered_json descending = nest(make_unordered_object(true), depth);
         const unordered_json ascending = nest(make_unordered_object(false), depth);
@@ -889,7 +889,7 @@ TEST_CASE("containers are compared element by element")
 
     for (const std::size_t depth : std::vector<std::size_t> {0, 200})
     {
-        CAPTURE(depth);
+        CAPTURE(depth)
 
         // objects with different keys
         {