[Modules] Implement an example module path for name lookup.

[iains]

Summary

With the ability to create compiled module interfaces 'on demand' when the
relevant keywords are encountered, we need some mechanism to determine the
output file name to be used for such modules.

In the most simplistic case, we can choose to name the module CMI based on
the name of the source file. However, in many cases that is likely to be an over-
simplification.

The intent is that this on-demand facility would be used with P1184 or some
similar scheme - but that can be abstracted behind some query of the the
module loader or module mapper.

As a proof-of-principle we re (maybe ab-)use the fmodule=name=path command line
options to look up the file path given the module name.

[iains]

reapply patch

[iains]

this will be updated and moved to a pull request

clang/lib/Serialization/GeneratePCH.cpp

void PCHGenerator::HandleTranslationUnit(ASTContext &Ctx) {
   if (IsForCMI && SemaPtr->isModuleImplementation())
     return;
 
   if (IsForCMI) {
 
     assert(Module && !Module->IsFromModuleFile &&
            "trying to re-write a module?");
 
+    // Here we would ideally use a P1184 server to find the module name.
+    // However, in the short-term we are going to (ab-)use the name/file pairs
+    // that can be specified with -fmodule-file=Name=Path.  If there is no
+    // entry there, then we fall back to the default CMI name, based on the
+    // source file name.
+    HeaderSearch &HS = PP.getHeaderSearchInfo();
+    const HeaderSearchOptions &HSOpts = HS.getHeaderSearchOpts();
+    std::string ModuleFilename;
+    if (!HSOpts.PrebuiltModuleFiles.empty() ||
+        !HSOpts.PrebuiltModulePaths.empty())
+      ModuleFilename = HS.getPrebuiltModuleFileName(Module->Name);
+
+    if (!ModuleFilename.empty())
+      OutputFile = ModuleFilename;
+
     // So now attach that name to the buffer we are about to create.
     Buffer->PresumedFileName = OutputFile;
   }
 
   Buffer->Signature =
       Writer.WriteAST(*SemaPtr, OutputFile, Module, isysroot,
                       // For serialization we are lenient if the errors were
                       // only warn-as-error kind.

clang/lib/Serialization/PCHContainerOperations.cpp

if (Buffer->IsComplete && !Buffer->PresumedFileName.empty()) {
       if (!EC) {
         int FD;
         EC = llvm::sys::fs::openFileForWrite(Buffer->PresumedFileName, FD);
         if (!EC) {
           std::unique_ptr<raw_pwrite_stream> OS;
           OS.reset(new llvm::raw_fd_ostream(FD, /*shouldClose=*/true));
           *OS << Buffer->Data;
           OS->flush(); // Make sure it hits disk now.
+          // Here we would notify P1184 servers that the module is created
         } else
           llvm::dbgs() << " Problem creating : " << Buffer->PresumedFileName
                        << "\n";
       } else
         llvm::dbgs() << " Problem creating dir : " << Parent << "\n";
     }
 
     // Free the space of the temporary buffer.