Preload standard library in ModuleInterfaceBuilder

Previously, when the standard library module interface was broken, Swift would try to rebuild it repeatedly during -compile-module-from-interface jobs because `ASTContext::getStdlibModule()` would try to load the standard library again each time it was called. This led to extremely slow compilations that repeatedly emitted the same errors.

To avoid this, we make ModuleInterfaceBuilder try to load the standard library right away and bail out if it can’t.

Fixes rdar://75669548.

lib/Frontend/ModuleInterfaceBuilder.cpp

@@ -187,6 +187,13 @@ bool ModuleInterfaceBuilder::buildSwiftModuleInternal(
     InputInfo.getPrimarySpecificPaths().SupplementaryOutputs;
     StringRef OutPath = OutputInfo.ModuleOutputPath;
 
+    // Bail out if we're going to use the standard library but can't load it. If
+    // we don't do this before we try to build the interface, we could end up
+    // trying to rebuild a broken standard library dozens of times due to
+    // multiple calls to `ASTContext::getStdlibModule()`.
+    if (SubInstance.loadStdlibIfNeeded())
+      return std::make_error_code(std::errc::not_supported);
+
     // Build the .swiftmodule; this is a _very_ abridged version of the logic
     // in performCompile in libFrontendTool, specialized, to just the one
     // module-serialization task we're trying to do here.