[clang][modules] Avoid using uninitialized SourceLocation

At this point, `F.ImportLoc` has not been initialized by the `ASTReader` yet and using it leads to an assertion failure.

Introduced in 638c673 and 4445135.

clang/lib/Serialization/ASTReader.cpp

@@ -2924,7 +2924,7 @@ ASTReader::ReadControlBlock(ModuleFile &F,
       // If we've already loaded a module map file covering this module, we may
       // have a better path for it (relative to the current build).
       Module *M = PP.getHeaderSearchInfo().lookupModule(
-          F.ModuleName, F.ImportLoc, /*AllowSearch*/ true,
+          F.ModuleName, SourceLocation(), /*AllowSearch*/ true,
           /*AllowExtraModuleMapSearch*/ true);
       if (M && M->Directory) {
         // If we're implicitly loading a module, the base directory can't