This removes the extractvalue constant expression, as part of https://discourse.llvm.org/t/rfc-remove-most-constant-expressions/63179. extractvalue is already not supported in bitcode, so we do not need to worry about auto-upgrade.
Uses of ConstantExpr::getExtractValue() should be replaced with IRBuilder::CreateExtractValue() (if the fact that the result is constant is not important) or ConstantFoldExtractValueInstruction() (if it is). Though for this particular case, it is also possible and probably a bit more elegant to use getAggregateElement() instead.
The C API function LLVMConstExtractValue() is removed, as the underlying constant expression no longer exists. Instead, LLVMBuildExtractValue() should be used (which will constant fold or create an instruction). Depending on the use-case, LLVMGetAggregateElement() may also be used instead.
Depends on D128213.