Update vendored LibFuzzer to LLVM 20.x release

[rchildre3]

Specifically:

• Tag: llvmorg-20.1.7
• Commit: 6146a88f60492b520a36f8f8f3231e15f3cc6082

Major changes:

• Clarified behavior of -max_len option

• Avoid UBSan faults in LibFuzzer by guarding memcpy() calls to check for valid size argument

• Fix building LibFuzzer with clang-cl

• Thread name setting logic for Fuchsia and Windows

Full commit log:

git log --format=ref release/19.x..release/20.x -- 'compiler-rt/lib/fuzzer/'

091741a880c2 ([libfuzzer] Clarify -max_len behavior on bigger files (#123095), 2025-01-23)
f9125ddc1faa (Revert "[libfuzzer] use timer_create() instead of setitimer() for linux" (#115811), 2024-11-11)
3b29a8a00809 ([libfuzzer] use timer_create() instead of setitimer() for linux (#110274), 2024-11-12)
5082acce4fd3 ([compiler-rt] Add custom libc++ workaround for CMake < 3.26, 2024-11-10)
87f4bc0acad6 ([compiler-rt] [fuzzer] Skip trying to set the thread name on MinGW (#115167), 2024-11-07)
e7bad34475e2 ([compiler-rt] Use installed libc++(abi) for tests instead of build tree, 2024-11-06)
a6fdfefbd04d ([compiler-rt] Include stdlib.h for exit() (#115025), 2024-11-05)
d54953ef472b ([fuzzer] fix clang-cl build fuzzer lit test failure (#112339), 2024-10-17)
b4130bee6bfd (Fix libFuzzer not building with pthreads on Windows (#109525), 2024-09-24)
b32dc677325c (Revert "[compiler-rt][fuzzer] SetThreadName build fix for Mingwin attempt (#106902)", 2024-09-02)
7c4cffd9d8be ([compiler-rt][fuzzer] SetThreadName build fix for Mingwin attempt (#106902), 2024-09-01)
f47966b1de45 ([compiler-rt] Reland "SetThreadName implementation for Fuchsia" (#105179), 2024-08-21)
ddaa8284f5b4 (Revert "[compiler-rt][fuzzer] implements SetThreadName for fuchsia." (#105162), 2024-08-20)
31cc4ccdea92 ([compiler-rt][fuzzer] implements SetThreadName for fuchsia. (#99953), 2024-08-20)
bde4ffe75214 (Don't pass null pointers to memcmp and memcpy in libFuzzer (#96775), 2024-08-13)
7202fe582931 ([compiler-rt] Silence warnings, 2024-08-11)

[fitzgen]

Thanks!