Switch to undefined #180

Job	Run time
abi-dump-setup	1m 21s
Test lld / Lit Tests (ubuntu-latest)	8m 42s
Build lldb / Lit Tests (ubuntu-latest)	24m 3s
Test llvm,clang,libclc / Lit Tests (ubuntu-latest)	43m 4s
Test lld / Lit Tests (windows-2019)	2m 25s
Build lldb / Lit Tests (windows-2019)	2m 25s
Test lld / Lit Tests (macOS-latest)	10m 30s
Test llvm,clang,libclc / Lit Tests (windows-2019)	1m 48s
Build lldb / Lit Tests (macOS-latest)	14m 56s
Test llvm,clang,libclc / Lit Tests (macOS-latest)	59m 39s
abi-dump (build-baseline)	30m 31s
abi-dump (build-latest)	30m 42s
abi-compare	30s
	3h 50m 36s

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