Started from Pauladam Smith's blog how to get started with llvm c api, then try to make it a comprehensive code guide about llvm c api

Typical cmake project, make sure LLVM in cmake search path

1. simple function -- create_sum_fn: Adopted Paladam's sum code to LLVM 14 and C++
2. call function in libc -- create_out_fn: Call functions from libc, puts
3. recursive calls and TCE -- create_factorial_fn: Recursive function factorial(n) with LLVMAddTailCallEliminationPass to eliminate recursive calls in IR
4. more complicated recursive calls and TCE -- create_fib_fn: Naive recursive implementation of fib(n), clearly only the last fib call in fib(n-1) + fib(n-2) got TCE optimized
5. if-the-else with PHI -- create_is_odd_fn: if then else pattern
6. while-do without PHI -- create_factorial_loop_fn: while-do without PHI. For factorial, this version and the recursive one optimized down to the same IR code.
7. array access with GEP -- create_ap_sum_fn: Sum of an array of arithmetic progression numbers, I really hoped LLVM can do it in the way of (f + l) * n / 2, but sadly no, still loops
8. Niubi LLVM optimization -- create_ap_simple_sum_fn: Sum of an array of a sequence of arithmetic progression numbers, still not using that formula, maybe because of some optimization are missing, unlike pure -O3, maybe it is being done during re-write

In orcv2jit.cpp file

1. Found memory leaks in llvm official example, filed a bug report, fixed in 4246269
2. In the same bug report, I mentioned LLVMOrcIRTransformLayerSetTransform causes memory leaks, fixed in bc062e0
3. cal_fact_4: call to another function in a different module