Transition to JITLink

Seeing what this will look like, since it has a number of features
(delayed compilation, concurrent compilation) that are starting to
become important, so it would be nice to switch to only supporting one
common implmenetation of memory management.

Refs #50248

I am expecting llvm/llvm-project#63236 may
cause problems, since we reconfigured some CI machines to hide that
issue, but it is still likely relevant.

src/Makefile

@@ -55,7 +55,7 @@ ifeq ($(JULIACODEGEN),LLVM)
 CODEGEN_SRCS := codegen jitlayers aotcompile debuginfo disasm llvm-simdloop llvm-muladd \
 	llvm-final-gc-lowering llvm-pass-helpers llvm-late-gc-lowering llvm-ptls \
 	llvm-lower-handlers llvm-gc-invariant-verifier llvm-propagate-addrspaces \
-	llvm-multiversioning llvm-alloc-opt llvm-alloc-helpers cgmemmgr llvm-remove-addrspaces \
+	llvm-multiversioning llvm-alloc-opt llvm-alloc-helpers llvm-remove-addrspaces \
 	llvm-remove-ni llvm-julia-licm llvm-demote-float16 llvm-cpufeatures pipeline llvm_api
 FLAGS += -I$(shell $(LLVM_CONFIG_HOST) --includedir)
 CG_LLVM_LIBS := all