[clang][extract-api] Don't emit symbols prefixed with an underscore

These symbols are understood to not be used for client API consumption
by convention so they should not appear in the generated symbol graph.

Differential Revision: https://reviews.llvm.org/D125678

Author: daniel-grumberg

clang/lib/ExtractAPI/Serialization/SymbolGraphSerializer.cpp

@@ -467,6 +467,11 @@ bool SymbolGraphSerializer::shouldSkip(const APIRecord &Record) const {
   if (Record.Availability.isUnconditionallyUnavailable())
     return true;
 
+  // Filter out symbols prefixed with an underscored as they are understood to
+  // be symbols clients should not use.
+  if (Record.Name.startswith("_"))
+    return true;
+
   return false;
 }