[cxxmodules] Don't complain when modulemap for implicit modules has changed

This patch (also) aims to make runtime module installable.

This part of code in Clang is comparing the location of "modulemap which
is currently loaded and gives a definition of current module (say, stl)
and "the location of the modulemap where the current implicit module (like stl) was built".

This was problematic for CMSSW, as they should install modulemaps
and prebuilt pcms to other directory. stl and libc pcms should be
prebuilt, installed and used from installed directory, so this check is
redundant for that usecase.

Author: vgvassilev

clang/lib/Serialization/ASTReader.cpp

@@ -3076,6 +3076,27 @@ ASTReader::ReadControlBlock(ModuleFile &F,
           return Missing;
         }
 
+        // Check if ImportedFile exists on disk
+        if (!llvm::sys::fs::is_directory(ImportedFile)) {
+          StringRef ModuleName = llvm::sys::path::filename(ImportedFile.c_str());
+          ModuleName.consume_back(".pcm");
+          // Get clang::Module pointer by looking up the module name
+          HeaderSearch &HS = PP.getHeaderSearchInfo();
+          Module *M = HS.lookupModule(ModuleName, SourceLocation(),
+                                      /*AllowSearch*/ true,
+                                      /*AllowExtraModuleMapSearch*/ true);
+          if (M) {
+            std::string Path = HS.getPrebuiltModuleFileName(M->Name);
+                           if (Path.empty())
+              Path = HS.getCachedModuleFileName(M->Name,
+                   HS.getModuleMap().getModuleMapFileForUniquing(M)->getName());
+            // FIXME: Add a hash comparison to check if ImportedFile's hash and the
+            // new Modules Path's hash matches or not.
+            if (!Path.empty())
+              ImportedFile = Path;
+          }
+        }
+
         // If our client can't cope with us being out of date, we can't cope with
         // our dependency being missing.
         unsigned Capabilities = ClientLoadCapabilities;
@@ -4066,11 +4087,31 @@ void ASTReader::ReadModuleOffsetMap(ModuleFile &F) const {
                           ? ModuleMgr.lookupByModuleName(Name)
                           : ModuleMgr.lookupByFileName(Name));
     if (!OM) {
-      std::string Msg =
-          "SourceLocation remap refers to unknown module, cannot find ";
-      Msg.append(std::string(Name));
-      Error(Msg);
-      return;
+      StringRef ModuleName = llvm::sys::path::filename(Name);
+      ModuleName.consume_back(".pcm");
+      HeaderSearch &HS = PP.getHeaderSearchInfo();
+      Module *M = HS.lookupModule(ModuleName, SourceLocation(),
+                                  /*AllowSearch*/ true,
+                                  /*AllowExtraModuleMapSearch*/ true);
+      std::string Path;
+      // If module definition exists in modulemap, search the modulepath in HeaderSearchInfo
+      if (M) {
+        Path = HS.getPrebuiltModuleFileName(M->Name);
+        if (Path.empty())
+          Path = HS.getCachedModuleFileName(M->Name,
+                   HS.getModuleMap().getModuleMapFileForUniquing(M)->getName());
+
+      }
+
+      StringRef NewName = StringRef(Path);
+      OM = ModuleMgr.lookupByFileName(NewName);
+      if (!OM) {
+        std::string Msg =
+           "SourceLocation remap refers to unknown module, cannot find ";
+        Msg.append(std::string(NewName));
+        Error(Msg);
+        return;
+      }
     }
 
     SourceLocation::UIntTy SLocOffset =