Reland "[Attr] support btf_type_tag attribute"

This is to revert commit f95bd18b (Revert "[Attr] support
btf_type_tag attribute") plus a bug fix.

Previous change failed to handle cases like below:
    $ cat reduced.c
    void a(*);
    void a() {}
    $ clang -c reduced.c -O2 -g

In such cases, during clang IR generation, for function a(),
CGCodeGen has numParams = 1 for FunctionType. But for
FunctionTypeLoc we have FuncTypeLoc.NumParams = 0. By using
FunctionType.numParams as the bound to access FuncTypeLoc
params, a random crash is triggered. The bug fix is to
check against FuncTypeLoc.NumParams before accessing
FuncTypeLoc.getParam(Idx).

Differential Revision: https://reviews.llvm.org/D111199