Use Builtin ID as the return value

Use Builtin ID as the return value
for FunctionDecl::getMemoryFunctionKind().

This is a follow up on the Chris's review for r148142: We don't want to
pollute FunctionDecl with an extra enum. (To make this work, added
memcmp and family to the library builtins.)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@148267 91177308-0d34-0410-b5e6-96231b3b80d8