Recommitting r275284: add support to inline __builtin_mempcpy

Patch by Sunita Marathe

Third try, now following fixes to MSan to handle mempcy in such a way that this commit won't break the MSan buildbots. (Thanks, Evegenii!)

llvm-svn: 277189
diff --git a/llvm/test/Transforms/InferFunctionAttrs/no-proto.ll b/llvm/test/Transforms/InferFunctionAttrs/no-proto.ll
index 256f5c3..25a4805 100644
--- a/llvm/test/Transforms/InferFunctionAttrs/no-proto.ll
+++ b/llvm/test/Transforms/InferFunctionAttrs/no-proto.ll
@@ -480,6 +480,9 @@
 ; CHECK: declare void @memcpy(...)
 declare void @memcpy(...)
 
+; CHECK: declare void @mempcpy(...)
+declare void @mempcpy(...)
+
 ; CHECK: declare void @memmove(...)
 declare void @memmove(...)