LTO: Check if symbol exists in closure before trying to copy it when externalizing

[giulianobelinassi]

When a library is compiled with LTO (-flto, link time optimization), we can not simply copy the symbol if it is not found in the debuginfo because the symbol may have been inlined and is not available in the current compilation unit.

An example of this is the symbol ssl3_release_read_buffer of libssl.so.1.1 on Factory repository. When extracting symbol ssl3_read_n of ssl/record/rec_layer_s3.c, the symbol ssl3_release_read_buffer is not externalized because the SymbolExternalizer correctly decides it was inlined, but since there is no body of this function in the compilation unit it just copies the prototype, which is incorrect.

Command line:

clang-extract -DCE_EXTRACT_FUNCTIONS=ssl3_read_n -DCE_OUTPUT_FILE=/tmp/out.c   -I. -Iinclude -fPIC -pthread -m64 -Wa,--noexecstack -Wall -O3 -O2 -Wall -U_FORTIFY_SOURCE -fstack-protector-strong -funwind-tables -fasynchronous-unwind-tables -fstack-clash-protection -Werror=return-type -flto=auto -g -fpatchable-function-entry=16,14 -fdump-ipa-clones -Wa,--noexecstack -fno-common -Wall -DOPENSSL_USE_NODELETE -DL_ENDIAN -DOPENSSL_PIC -DOPENSSL_CPUID_OBJ -DOPENSSL_IA32_SSE2 -DOPENSSL_BN_ASM_MONT -DOPENSSL_BN_ASM_MONT5 -DOPENSSL_BN_ASM_GF2m -DSHA1_ASM -DSHA256_ASM -DSHA512_ASM -DKECCAK1600_ASM -DRC4_ASM -DMD5_ASM -DAESNI_ASM -DVPAES_ASM -DGHASH_ASM -DECP_NISTZ256_ASM -DX25519_ASM -DPOLY1305_ASM -DOPENSSLDIR="\"/etc/ssl\"" -DENGINESDIR="\"/usr/lib64/engines-1.1\"" -DNDEBUG -D_FORTIFY_SOURCE=3 -DTERMIO -DPURIFY -D_GNU_SOURCE -DOPENSSL_NO_BUF_FREELISTS -DSYSTEM_CIPHERS_FILE="\"/etc/crypto-policies/back-ends/openssl.config\"" -MMD -MF ssl/record/rec_layer_s3.d.tmp -MT ssl/record/rec_layer_s3.o -c -o ssl/record/rec_layer_s3.o ssl/record/rec_layer_s3.c