[TLI] Robustize SDAG LibFunc proto checking by merging it into TLI.

This re-applies commit r292189, reverted in r292191.

SelectionDAGBuilder recognizes libfuncs using some homegrown
parameter type-checking.

Use TLI instead, removing another heap of redundant code.

This isn't strictly NFC, as the SDAG code was too lax.
Concretely, this means changes are required to a few tests:
- calling a non-variadic function via a variadic prototype isn't OK;
  it just happens to work on x86_64 (but not on, e.g., aarch64).
- mempcpy has a size_t parameter;  the SDAG code accepts any integer
  type, which meant using i32 on x86_64 worked.
- a handful of SystemZ tests check the SDAG support for lax prototype
  checking: Ulrich agrees on removing them.

I don't think it's worth supporting any of these (IMO) invalid
testcases.  Instead, fix them to be more meaningful.

llvm-svn: 294028