[BOLT] Force non-relocation mode for heatmap generation

Summary: BOLT operates in relocation mode by default when .reloc is in the binary. This changes disables relocation mode for heatmap generation so we can use that for more cases. There's a small separate change that ignores zero-sized symbol in zero-sized code section during function discovery.

(cherry picked from FBD16009610)

bolt/src/RewriteInstance.cpp

@@ -1486,6 +1486,11 @@ void RewriteInstance::discoverFileObjects() {
       BF->addAlternativeName(UniqueName);
     } else {
       auto Section = BC->getSectionForAddress(Address);
+
+      // Skip zero-size symbol in zero-size section
+      if (!Section && !SymbolSize)
+        continue;
+
       assert(Section && "section for functions must be registered.");
       BF = BC->createBinaryFunction(UniqueName, *Section, Address,
                                     SymbolSize, IsSimple);
@@ -1868,6 +1873,12 @@ void RewriteInstance::readSpecialSections() {
 
   BC->HasRelocations = HasTextRelocations &&
                        (opts::RelocationMode != cl::BOU_FALSE);
+
+  // Force non-relocation mode for heatmap generation
+  if (opts::HeatmapMode) {
+    BC->HasRelocations = false;
+  }
+
   if (BC->HasRelocations) {
     outs() << "BOLT-INFO: enabling relocation mode\n";
   }