[publish images] Upgrade LLVM toolchain from 19.1.7 to 21.1.8 (#26667)

## Summary
- Update LLVM version references across build scripts, Dockerfiles, CI,
Nix configs, and documentation
- Fix LLVM 21 `-Wcharacter-conversion` errors in WebKit bindings:
- `EncodingTables.h`: pragma for intentional char32_t/char16_t
comparisons
- `TextCodecCJK.cpp`: widen `gb18030AsymmetricEncode` param to char32_t
- `URLPatternParser`: widen `isValidNameCodepoint` param to char32_t,
cast for `startsWith`
- Fix `__libcpp_verbose_abort` noexcept mismatch (LLVM 21 uses
`_NOEXCEPT`)
- Fix dangling pointer in `BunJSCModule.h` (`toCString` temporary
lifetime)
- Remove `useMathSumPreciseMethod` (removed upstream in JSC)

**Before merging:** Merge https://github.com/oven-sh/WebKit/pull/153
first, then update `WEBKIT_VERSION` in `cmake/tools/SetupWebKit.cmake`
to point to the merged commit.

## Test plan
- [ ] Build bun debug on macOS with LLVM 21
- [ ] Build bun on Linux (glibc)
- [ ] Build bun on Linux (musl)
- [ ] Build bun on Windows
- [ ] Run test suite

Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

src/bun.js/bindings/webcore/URLPatternParser.cpp

@@ -470,7 +470,7 @@ String generatePatternString(const Vector<Part>& partList, const URLPatternStrin
 
         if (!needsGrouping && part.prefix.isEmpty() && previousPart && previousPart->type == PartType::FixedText && !previousPart->value.isEmpty()) {
             if (options.prefixCodepoint.length() == 1
-                && options.prefixCodepoint.startsWith(*StringView(previousPart->value).codePoints().codePointAt(previousPart->value.length() - 1)))
+                && options.prefixCodepoint.startsWith(static_cast<char16_t>(*StringView(previousPart->value).codePoints().codePointAt(previousPart->value.length() - 1))))
                 needsGrouping = true;
         }
 
@@ -541,7 +541,7 @@ String escapePatternString(StringView input)
 }
 
 // https://urlpattern.spec.whatwg.org/#is-a-valid-name-code-point
-bool isValidNameCodepoint(char16_t codepoint, URLPatternUtilities::IsFirst first)
+bool isValidNameCodepoint(char32_t codepoint, URLPatternUtilities::IsFirst first)
 {
     if (first == URLPatternUtilities::IsFirst::Yes)
         return u_hasBinaryProperty(codepoint, UCHAR_ID_START) || codepoint == '_' || codepoint == '$';