[DebugInfo] Change DIEnumerator payload type from int64_t to APInt

This allows the representation of arbitrarily large enumeration values.
See https://lists.llvm.org/pipermail/llvm-dev/2017-December/119475.html for context.

Reviewed By: andrewrk, aprantl, MaskRay

Differential Revision: https://reviews.llvm.org/D62475

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -2317,7 +2317,7 @@ Error BitcodeReader::resolveGlobalAndIndirectSymbolInits() {
   return Error::success();
 }
 
-static APInt readWideAPInt(ArrayRef<uint64_t> Vals, unsigned TypeBits) {
+APInt llvm::readWideAPInt(ArrayRef<uint64_t> Vals, unsigned TypeBits) {
   SmallVector<uint64_t, 8> Words(Vals.size());
   transform(Vals, Words.begin(),
                  BitcodeReader::decodeSignRotatedValue);