[flang][driver] Make the -J option less restrictive so we would not have to struggle with autoconf (#110010)

There are autoconf-configured projects for which the generated Makefile
is invoking flang with more than one -J option, each one specifying the
same directory. Although only one module directory should be specified
(by either -J or -module-dir), it should not really matter how many
times this same directory has been specified.

Apparently, other compilers understand it that way, hence autoconf's
configure script may generate a Makefile with the repetitive -J's.

For example, when trying to build the ABINIT [1] project (which can be
configured by either CMake or the configure script) when configured by
autoconf, it fails to build as such:

```
make[3]: Entering directory 'src/98_main'
mpifort -DHAVE_CONFIG_H -I. -I../../../src/98_main -I../..  -I../../src/incs -I../../../src/incs -Ifallbacks/exports/include  -Jbuild/mods  -Jbuild/mods -c -o abinit-abinit.o `test -f 'abinit.F90' || echo '../../../src/98_main/'`abinit.F90
error: Only one '-module-dir/-J' option allowed
make[3]: *** [Makefile:3961: abinit-abinit.o] Error 1
```

This patch solves the problem.

[1] https://github.com/abinit/abinit.git

1 files changed, 10 insertions, 3 deletions

diff --git a/flang/lib/Frontend/CompilerInvocation.cpp b/flang/lib/Frontend/CompilerInvocation.cpp
index 05b03ba..d68534d 100644
--- a/flang/lib/Frontend/CompilerInvocation.cpp
+++ b/flang/lib/Frontend/CompilerInvocation.cpp
@@ -40,6 +40,7 @@
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/TargetParser/Host.h"
 #include "llvm/TargetParser/Triple.h"
+#include <algorithm>
 #include <cstdlib>
 #include <memory>
 #include <optional>
@@ -830,14 +831,20 @@ static bool parseSemaArgs(CompilerInvocation &res, llvm::opt::ArgList &args,
   unsigned numErrorsBefore = diags.getNumErrors();
 
   // -J/module-dir option
-  auto moduleDirList =
+  std::vector<std::string> moduleDirList =
       args.getAllArgValues(clang::driver::options::OPT_module_dir);
-  // User can only specify -J/-module-dir once
+  // User can only specify one -J/-module-dir directory, but may repeat
+  // -J/-module-dir as long as the directory is the same each time.
   // https://gcc.gnu.org/onlinedocs/gfortran/Directory-Options.html
+  std::sort(moduleDirList.begin(), moduleDirList.end());
+  moduleDirList.erase(std::unique(moduleDirList.begin(), moduleDirList.end()),
+                      moduleDirList.end());
   if (moduleDirList.size() > 1) {
     const unsigned diagID =
         diags.getCustomDiagID(clang::DiagnosticsEngine::Error,
-                              "Only one '-module-dir/-J' option allowed");
+                              "Only one '-module-dir/-J' directory allowed. "
+                              "'-module-dir/-J' may be given multiple times "
+                              "but the directory must be the same each time.");
     diags.Report(diagID);
   }
   if (moduleDirList.size() == 1)