[CGBuiltin] Respect asm labels and redefine_extname for builtins with specialized emitting

rL131311 added `asm()` support for builtin functions, but `asm()` for builtins with
specialized emitting (e.g. memcpy, various math functions) still do not work.

This patch makes these functions work for `asm()` and `#pragma redefine_extname`.
glibc uses `asm()` to redirect internal libc function calls to hidden aliases.

Limitation: such a function is a builtin in clang, but will not be recognized as
a libcall in optimization passes because Clang does not annotate the renamed
function as a libcall.  In GCC -O1 or above, `abs` can be optimized out but we can't.
Additionally, we cannot redirect `__builtin_sin` to `real_sin` in the following example:

  double sin(double x) asm("real_sin");
  double f(double d) { return __builtin_sin(d); }

---

According to @rsmith, the following three statements cannot be simultaneously true:

(1) The frontend function foo has known, builtin semantics X.
(2) The symbol foo has known, builtin semantics X.
(3) It's not correct to lower a call to the frontend function foo to the symbol foo.

People do want (1) (if it is profitable to expand a memcpy, do it).
This also means that people do not want to add -fno-builtin-memcpy.
People do want (3): that is why they use asm("__GI_memcpy") in the first place.

So unfortunately we make a compromise by not refuting (2) (see the limitation above).
For most libcalls, there is a small loss because compilers don't synthesize them.
For the few glibc cares about, it uses `asm("memcpy = __GI_memcpy");` to make
the assembly level redirection.
(Changing function names (e.g. `__memcpy`) is a hit to ergonomics which is not acceptable).

Reviewed By: rsmith

Differential Revision: https://reviews.llvm.org/D88712

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