[IR] Adds mustprogress as a LLVM IR attribute

This adds the LLVM IR attribute `mustprogress` as defined in LangRef through D86233. This attribute will be applied to functions with in languages like C++ where forward progress is guaranteed. Functions without this attribute are not required to make progress.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D85393

llvm/lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -743,6 +743,8 @@ static uint64_t getAttrKindEncoding(Attribute::AttrKind Kind) {
     return bitc::ATTR_KIND_NOUNDEF;
   case Attribute::ByRef:
     return bitc::ATTR_KIND_BYREF;
+  case Attribute::MustProgress:
+    return bitc::ATTR_KIND_MUSTPROGRESS;
   case Attribute::EndAttrKinds:
     llvm_unreachable("Can not encode end-attribute kinds marker.");
   case Attribute::None: