Porting Clang rC324498 to Emscripten.

This change adds option to manually control whether to discard value
names in LLVM IRs generated by Clang.

Upstream change can be viewed at: https://reviews.llvm.org/rC324498

Author: milizhang

include/clang/Driver/Options.td

@@ -786,6 +786,10 @@ def fdiagnostics_show_template_tree : Flag<["-"], "fdiagnostics-show-template-tr
     HelpText<"Print a template comparison tree for differing templates">;
 def fdeclspec : Flag<["-"], "fdeclspec">, Group<f_clang_Group>,
   HelpText<"Allow __declspec as a keyword">, Flags<[CC1Option]>;
+def fdiscard_value_names : Flag<["-"], "fdiscard-value-names">, Group<f_clang_Group>,
+  HelpText<"Discard value names in LLVM IR">, Flags<[DriverOption]>;
+def fno_discard_value_names : Flag<["-"], "fno-discard-value-names">, Group<f_clang_Group>,
+  HelpText<"Do not discard value names in LLVM IR">, Flags<[DriverOption]>;
 def fdollars_in_identifiers : Flag<["-"], "fdollars-in-identifiers">, Group<f_Group>,
   HelpText<"Allow '$' in identifiers">, Flags<[CC1Option]>;
 def fdwarf2_cfi_asm : Flag<["-"], "fdwarf2-cfi-asm">, Group<clang_ignored_f_Group>;

lib/Driver/ToolChains/Clang.cpp

@@ -3226,11 +3226,24 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
 
 // Disable the verification pass in -asserts builds.
 #ifdef NDEBUG
-  CmdArgs.push_back("-disable-llvm-verifier");
-  // Discard LLVM value names in -asserts builds.
-  CmdArgs.push_back("-discard-value-names");
+  const bool IsAssertBuild = false;
+#else
+  const bool IsAssertBuild = true;
 #endif
 
+  // Disable the verification pass in -asserts builds.
+  if (!IsAssertBuild)
+    CmdArgs.push_back("-disable-llvm-verifier");
+
+  // Discard value names in assert builds unless otherwise specified.
+  if (const Arg *A = Args.getLastArg(options::OPT_fdiscard_value_names,
+                                     options::OPT_fno_discard_value_names)) {
+    if (A->getOption().matches(options::OPT_fdiscard_value_names))
+      CmdArgs.push_back("-discard-value-names");
+  } else if (!IsAssertBuild)
+    CmdArgs.push_back("-discard-value-names");
+
+
   // Set the main file name, so that debug info works even with
   // -save-temps.
   CmdArgs.push_back("-main-file-name");