[Bitcode] Address backwards compat bug in r342631

r342631 expanded bitc::METADATA_LOCATION by one element. The bitcode
metadata loader was changed in a backwards-incompatible way, leading to
crashes when disassembling old bitcode:

  assertion: empty() && "PlaceholderQueue hasn't been flushed before being destroyed"
  Assertion failed: (empty() && "PlaceholderQueue hasn't been flushed before being destroyed")

This commit teaches the metadata loader to assume that the newly-added
IsImplicitCode bit is 'false' when not present in old bitcode. I've added a
bitcode compat regression test.

rdar://44645820

llvm-svn: 342678

lib/Bitcode/Reader/MetadataLoader.cpp

@@ -1139,7 +1139,7 @@ Error MetadataLoader::MetadataLoaderImpl::parseOneMetadata(
     break;
   }
   case bitc::METADATA_LOCATION: {
-    if (Record.size() != 6)
+    if (Record.size() != 5 && Record.size() != 6)
       return error("Invalid record");
 
     IsDistinct = Record[0];
@@ -1147,7 +1147,7 @@ Error MetadataLoader::MetadataLoaderImpl::parseOneMetadata(
     unsigned Column = Record[2];
     Metadata *Scope = getMD(Record[3]);
     Metadata *InlinedAt = getMDOrNull(Record[4]);
-    bool ImplicitCode = Record[5];
+    bool ImplicitCode = Record.size() == 6 && Record[5];
     MetadataList.assignValue(
         GET_OR_DISTINCT(DILocation, (Context, Line, Column, Scope, InlinedAt,
                                      ImplicitCode)),