Update toolchain to LLVM 18 #281

Job	Run time
abi-dump-setup	1m 36s
Test llvm,clang,libclc / Lit Tests (ubuntu-latest)	5m 54s
Build lldb / Lit Tests (ubuntu-latest)	5m 48s
Test llvm,clang,libclc / Lit Tests (windows-2019)	2m 16s
Build lldb / Lit Tests (windows-2019)	2m 3s
Test llvm,clang,libclc / Lit Tests (macOS-latest)	4m 56s
Build lldb / Lit Tests (macOS-latest)	4m 54s
Test lld / Lit Tests (ubuntu-latest)	5m 51s
Test lld / Lit Tests (windows-2019)	2m 6s
Test lld / Lit Tests (macOS-latest)	4m 51s
abi-dump (build-baseline)	3m 9s
abi-dump (build-latest)	3m 9s
abi-compare	1s
	46m 34s

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