[Attributor] Deduce "nosync" function attribute.

Introduce and deduce "nosync" function attribute to indicate that a function
does not synchronize with another thread in a way that other thread might free memory.

Reviewers: jdoerfert, jfb, nhaehnle, arsenm

Subscribers: wdng, hfinkel, nhaenhle, mehdi_amini, steven_wu,
dexonsmith, arsenm, uenoku, hiraditya, jfb, llvm-commits

Differential Revision: https://reviews.llvm.org/D62766

llvm-svn: 365830

lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -659,6 +659,8 @@ static uint64_t getAttrKindEncoding(Attribute::AttrKind Kind) {
     return bitc::ATTR_KIND_NO_RED_ZONE;
   case Attribute::NoReturn:
     return bitc::ATTR_KIND_NO_RETURN;
+  case Attribute::NoSync:
+    return bitc::ATTR_KIND_NOSYNC;
   case Attribute::NoCfCheck:
     return bitc::ATTR_KIND_NOCF_CHECK;
   case Attribute::NoUnwind: