[Serialization] Improve module loading performance

When looking for a Swift module on disk, we were scanning all module search paths if they contain the module we are searching for. In a setup where each module is contained in its own framework search path, this scaled quadratically with the number of modules being imported. E.g. a setup with 100 modules being imported form 100 module search paths could cause on the order of 10,000 checks of `FileSystem::exists`. While these checks are fairly fast (~10µs), they add up to ~100ms.

To improve this, perform a first scan of all module search paths and list the files they contain. From this, create a lookup map that maps filenames to the search paths they can be found in. E.g. for
```
searchPath1/
  Module1.framework

searchPath2/
  Module1.framework
  Module2.swiftmodule
```
we create the following lookup table
```
Module1.framework -> [searchPath1, searchPath2]
Module2.swiftmodule -> [searchPath2]
```

lib/Sema/SourceLoader.cpp

@@ -45,7 +45,7 @@ static FileOrError findModule(ASTContext &ctx, Identifier moduleID,
   // should be searched.
   StringRef moduleNameRef = ctx.getRealModuleName(moduleID).str();
 
-  for (auto Path : ctx.SearchPathOpts.ImportSearchPaths) {
+  for (const auto &Path : ctx.SearchPathOpts.getImportSearchPaths()) {
     inputFilename = Path;
     llvm::sys::path::append(inputFilename, moduleNameRef);
     inputFilename.append(".swift");