Driver: Accept multiple --config options if filenames are the same

[tstellar]

Summary

We're trying to use the --config options to pass distro specific
options for Fedora via the CFLAGS variable. However, some projects
end up using the CFLAGS variable multiple times in their command line,
which leads to an error when --config is used.

This patch resolves this issue by allowing more than one --config option
on the command line as long as the file names are the same.

[sepavloff]

LGTM.

clang/lib/Driver/Driver.cpp

if (CLOptions) {
     }
   }
 
   // First try to find config file specified in command line.
   if (CLOptions) {
     std::vector<std::string> ConfigFiles =
         CLOptions->getAllArgValues(options::OPT_config);
     if (ConfigFiles.size() > 1) {
+      if (!std::all_of(
+              ConfigFiles.begin(), ConfigFiles.end(),
+              [ConfigFiles](std::string s) { return s == ConfigFiles[0]; })) {
         Diag(diag::err_drv_duplicate_config);
         return true;
       }
+    }
 
     if (!ConfigFiles.empty()) {
       CfgFileName = ConfigFiles.front();
       assert(!CfgFileName.empty());
 
       // If argument contains directory separator, treat it as a path to
       // configuration file.
       if (llvm::sys::path::has_parent_path(CfgFileName)) {

clang/test/Driver/config-file.c

 // CHECK-UNUSED-NOT: 'linker' input unused
 
 
 //--- User directory is searched first.
 //
 // RUN: %clang --config-system-dir=%S/Inputs/config --config-user-dir=%S/Inputs/config2 --config config-4 -S %s -o /dev/null -v 2>&1 | FileCheck %s -check-prefix CHECK-PRECEDENCE
 // CHECK-PRECEDENCE: Configuration file: {{.*}}Inputs{{.}}config2{{.}}config-4.cfg
 // CHECK-PRECEDENCE: -Wall
+
+
+//--- Duplicate --config options are allowed if the value is the same
+// RUN: %clang --config-system-dir=%S/Inputs/config --config-user-dir=%S/Inputs/config2 --config config-4 --config config-4 -S %s -o /dev/null -v 2>&1 | FileCheck %s -check-prefix CHECK-SAME-CONFIG
+// CHECK-SAME-CONFIG: Configuration file: {{.*}}Inputs{{.}}config2{{.}}config-4.cfg