Remove `LLVMExtSetCurrentDebugLocation` from `llvm_ext.cc` for LLVM 9… #2504

Job	Run time
check_format	1m 46s
x86_64-gnu-test (1.7.3)	41m 50s
x86_64-gnu-test (1.8.2)	29m 31s
x86_64-gnu-test (1.9.2)	47m 52s
x86_64-gnu-test (1.10.1)	30m 30s
x86_64-gnu-test (1.0.0, FLAGS=-Dwithout_ffi USE_PCRE1=true)	29m 55s
x86_64-gnu-test (1.1.1, FLAGS=-Dwithout_ffi USE_PCRE1=true)	37m 19s
x86_64-gnu-test (1.2.2, USE_PCRE1=true)	46m 11s
x86_64-gnu-test (1.3.2, USE_PCRE1=true)	40m 11s
x86_64-gnu-test (1.4.1, USE_PCRE1=true)	31m 16s
x86_64-gnu-test (1.5.1, USE_PCRE1=true)	29m 43s
x86_64-gnu-test (1.6.2, USE_PCRE1=true)	42m 52s
x86_64-musl-test	52m 50s
x86_64-gnu-test-preview_mt	8m 33s
deploy_api_docs	0s
	7h 50m 19s

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