llvm: upgrade 18.1.5 -> 18.1.6

Changelog:
==========
- Fixes issues where LLVM is either generating the incorrect thunk for a
  function with aligned parameters or didn't correctly pass through the
  return value when StructRet was used.
- -Xclang -target-feature -Xclang +unaligned-scalar-mem can be used to enable
  unaligned scalar memory accesses for CPUs that do not support unaligned
  vector accesses. -mno-strict-align will enable unaligned scalar and vector
  memory accesses.
- Don't replace an aliasee with an alias that has weak linkage. This avoids
  incorrect linkage that can lead to using the wrong symbols during linking time.
- This patch fixes build failures when compiling AVX512 code using
  -march=native on machines without AVX512.
- Fixes crash in AArch64 backend when having true or false as operand for a
  fcmp instruction on IR level.
- Fixes compiler crash when user specifies -mno-evex512 with AVX512 features
  but no AVX512VL.
- Fixes a bug that tries to do VBROADCAST_LOAD for f16 without AVX2.

Signed-off-by: Wang Mingyu <wangmy@fujitsu.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>