Remove `LLVMExtSetCurrentDebugLocation` from `llvm_ext.cc` for LLVM 9+ #9887

Job	Run time
check_format	1m 52s
x86_64-gnu-test (1.7.3)	31m 5s
x86_64-musl-test	43m 28s
x86_64-gnu-test (1.8.2)	35m 34s
x86_64-gnu-test-preview_mt	5m 6s
x86_64-gnu-test (1.9.2)	39m 12s
x86_64-gnu-test (1.10.1)	29m 17s
x86_64-gnu-test (1.0.0, FLAGS=-Dwithout_ffi USE_PCRE1=true)	29m 26s
x86_64-gnu-test (1.1.1, FLAGS=-Dwithout_ffi USE_PCRE1=true)	31m 52s
x86_64-gnu-test (1.2.2, USE_PCRE1=true)	47m 20s
deploy_api_docs	0s
x86_64-gnu-test (1.3.2, USE_PCRE1=true)	56m 9s
x86_64-gnu-test (1.4.1, USE_PCRE1=true)	55m 7s
x86_64-gnu-test (1.5.1, USE_PCRE1=true)	50m 39s
x86_64-gnu-test (1.6.2, USE_PCRE1=true)	41m 0s
	8h 17m 7s

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