processInterproceduralReferences: record references to cold fragments as entry points

Summary:
For interprocedural references to fragments, record them as
fragment entry points. Not registering these entry points leads to
UCE removing the blocks and "Undefined temporary symbol"
assertion.

(cherry picked from FBD24511281)

bolt/src/BinaryContext.cpp

@@ -1049,12 +1049,10 @@ void BinaryContext::processInterproceduralReferences(BinaryFunction &Function) {
       continue;
 
     if (TargetFunction) {
-      if (TargetFunction->IsFragment) {
+      if (TargetFunction->IsFragment)
         registerFragment(*TargetFunction, Function);
-      } else if (TargetFunction->getAddress() != Address) {
-        TargetFunction->
-          addEntryPointAtOffset(Address - TargetFunction->getAddress());
-      }
+      if (auto Offset = Address - TargetFunction->getAddress())
+        TargetFunction->addEntryPointAtOffset(Offset);
 
       continue;
     }

bolt/test/X86/interprocedural_ref_entry_point.s

@@ -0,0 +1,51 @@
+# This reproduces a bug where not registering cold fragment entry points 
+# leads to removing blocks and an inconsistent CFG after UCE.
+# Test assembly was obtained using C-Reduce from this C++ code:
+# (compiled with `g++ -O2 -Wl,-q`)
+#
+# #include <stdexcept>
+# int a;
+# int main() {
+#  if (a)
+#    try {
+#      throw std::logic_error("");
+#    } catch (...) {}
+#  try {
+#    throw std::logic_error("");
+#  } catch (...) {}
+# }
+
+# REQUIRES: system-linux
+
+# RUN: llvm-mc -filetype=obj -triple x86_64-unknown-unknown %s -o %t.o
+# RUN: %host_cxx %t.o -o %t.exe -Wl,-q
+# RUN: llvm-bolt %t.exe -o %t.out -dump-dot-all -funcs=main.* 2>&1 | FileCheck %s
+#
+# CHECK-NOT: Assertion `isValid()' failed.
+
+	.globl main
+main:
+	jmp .L3 # jump to the secondary entry point in main.cold.0
+
+	.section a,"ax"
+main.cold.0:
+	.cfi_startproc
+	.cfi_lsda 3,b
+  ud2
+  call __cxa_throw
+.L3:
+  nop
+	.cfi_endproc
+
+	.section  .gcc_except_table
+b:
+  .byte 0xff,0x3
+  .uleb128 e-c
+c:
+  .byte 1
+  .uleb128 e-d
+d:
+  .uleb128 0,0,0,0,0
+  .uleb128 .L3-main.cold.0
+  .uleb128 .L3-main.cold.0
+e:

bolt/test/lit.cfg.py

@@ -42,6 +42,7 @@ config.test_exec_root = os.path.join(config.bolt_obj_root, 'test')
 llvm_config.use_default_substitutions()
 
 config.substitutions.append(('%host_cc', config.host_cc))
+config.substitutions.append(('%host_cxx', config.host_cxx))
 
 tool_dirs = [config.llvm_tools_dir,
              config.test_source_root]