[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/Serialization/ModuleFile.cpp

@@ -165,7 +165,7 @@ Status ModuleFile::associateWithFileContext(FileUnit *file, SourceLoc diagLoc,
     return error(status);
   }
 
-  StringRef SDKPath = ctx.SearchPathOpts.SDKPath;
+  StringRef SDKPath = ctx.SearchPathOpts.getSDKPath();
   if (SDKPath.empty() ||
       !Core->ModuleInputBuffer->getBufferIdentifier().startswith(SDKPath)) {
     for (const auto &searchPath : Core->SearchPaths) {