[BOLT] Emit bolt-synthetic FILE symbol

Follow-up to llvm#89648.
Emit a FILE symbol to distinguish BOLT-created local symbols for split
functions, to prevent polluting the namespace of the last file symbol.

Test Plan: Updated cdsplit-symbol-names.s

Author: aaupov

bolt/lib/Rewrite/RewriteInstance.cpp

@@ -4493,6 +4493,17 @@ void RewriteInstance::updateELFSymbolTable(
   // Symbols for the new symbol table.
   std::vector<ELFSymTy> Symbols;
 
+  // Prepend synthetic FILE symbol to prevent local cold fragments from
+  // colliding with existing symbols with the same name.
+  ELFSymTy FileSymbol;
+  FileSymbol.st_shndx =
+      BC->getUniqueSectionByName(BC->getColdCodeSectionName())->getIndex();
+  FileSymbol.st_name = AddToStrTab("bolt-synthetic");
+  FileSymbol.st_value = 0;
+  FileSymbol.st_size = 0;
+  FileSymbol.setBindingAndType(ELF::STB_LOCAL, ELF::STT_FILE);
+  Symbols.emplace_back(FileSymbol);
+
   auto getNewSectionIndex = [&](uint32_t OldIndex) {
     // For dynamic symbol table, the section index could be wrong on the input,
     // and its value is ignored by the runtime if it's different from

bolt/test/X86/cdsplit-symbol-names.s

@@ -10,6 +10,7 @@
 # RUN:         --call-scale=2 --data=%t.fdata --reorder-blocks=ext-tsp
 # RUN: llvm-objdump --syms %t.bolt | FileCheck %s --check-prefix=CHECK-SYMS-WARM
 
+# CHECK-SYMS-WARM: [[#%X,VALUE:0]] l df *ABS* [[#%X,SIZE:0]] bolt-synthetic
 # CHECK-SYMS-WARM: .text.warm
 # CHECK-SYMS-WARM-SAME: chain.warm
 # CHECK-SYMS-WARM: .text.cold