TLI: Darwin sometimes does not support _bcmp, so do not emit it for Darwin

[arphaman]

Summary

Not all Darwin targets support _bcmp in all circumstances. Unfortunately after https://reviews.llvm.org/D56593 clang started emitting _bcmp for all Darwin targets. This patch reverts the previous behavior for Darwin.

[jfb]

I think importantly, even if Darwin has it it seems to be defined something like this:
https://github.com/apple/darwin-libplatform/blob/51ce256579c9b879704c868c9cd906ae54f733b5/src/string/generic/memcmp.c#L58

llvm/trunk/lib/Analysis/TargetLibraryInfo.cpp

 
 static bool hasBcmp(const Triple &TT) {
   // Posix removed support from bcmp() in 2001, but the glibc and several
   // implementations of the libc still have it.
   if (TT.isOSLinux())
     return TT.isGNUEnvironment() || TT.isMusl();
   // Both NetBSD and OpenBSD are planning to remove the function. Windows does
   // not have it.
-  return TT.isOSFreeBSD() || TT.isOSSolaris() || TT.isOSDarwin();
+  return TT.isOSFreeBSD() || TT.isOSSolaris();
 }
 
 /// Initialize the set of available library functions based on the specified
 /// target triple. This should be carefully written so that a missing target
 /// triple gets a sane set of defaults.
 static void initialize(TargetLibraryInfoImpl &TLI, const Triple &T,
                        ArrayRef<StringRef> StandardNames) {
   // Verify that the StandardNames array is in alphabetical order.

llvm/trunk/test/Transforms/InferFunctionAttrs/annotate.ll

 declare i32 @atoi(i8*)
 
 ; CHECK: declare i64 @atol(i8* nocapture) [[G2]]
 declare i64 @atol(i8*)
 
 ; CHECK: declare i64 @atoll(i8* nocapture) [[G2]]
 declare i64 @atoll(i8*)
 
-; CHECK-DARWIN: declare i32 @bcmp(i8* nocapture, i8* nocapture, i64) [[G2]]
 ; CHECK-LINUX: declare i32 @bcmp(i8* nocapture, i8* nocapture, i64) [[G2]]
+; CHECK-DARWIN-NOT: declare i32 @bcmp(i8* nocapture, i8* nocapture, i64) [[G2]]
 ; CHECK-UNKNOWN-NOT: declare i32 @bcmp(i8* nocapture, i8* nocapture, i64) [[G2]]
 ; CHECK-NVPTX-NOT: declare i32 @bcmp(i8* nocapture, i8* nocapture, i64) [[G2]]
 declare i32 @bcmp(i8*, i8*, i64)
 
 ; CHECK: declare void @bcopy(i8* nocapture readonly, i8* nocapture, i64) [[G1]]
 declare void @bcopy(i8*, i8*, i64)
 
 ; CHECK: declare void @bzero(i8* nocapture, i64) [[G1]]