Implement semantic checking for __builtin_signbit.

r242675 changed the signature for the signbit builtin but did not introduce proper semantic checking to ensure the arguments are as-expected. This patch groups the signbit builtin along with the other fp classification builtins. Fixes PR28172.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@335048 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed