[Modules] Soft reset diagnostics when clang module import failed

Swift compiler can try to import clang modules which might fail to load.
In this case, the clang diagnostic engine is reset to ignore the errors,
but the hard reset also clears out the states, including the diagnostics
options and current states. This will affect the future clang module
imports and diagnostics. Use a soft reset that only clears the errors
but not the other states.

rdar://139723218

Author: cachemeifyoucan

lib/ClangImporter/ClangDiagnosticConsumer.h

@@ -53,16 +53,18 @@ class ClangDiagnosticConsumer : public clang::TextDiagnosticPrinter {
     ~LoadModuleRAII() {
       if (Consumer) {
         // We must reset Clang's diagnostic engine here since we know that only
-        // the module lookup errors have been emitted. While the ClangDiagnosticConsumer
-        // takes care of filtering out the diagnostics from the output and from
-        // being handled by Swift's DiagnosticEngine, we must ensure to also
-        // reset Clang's DiagnosticEngine because its state is queried in later
-        // stages of compilation and errors emitted on "module_not_found" should not
-        // be counted.
-        // FIXME: We must instead allow for module loading in Clang to fail without
-        // needing to emit a diagnostic.
-        if (Engine && Consumer->CurrentImportNotFound && DiagEngineClearPriorToLookup)
-            Engine->Reset();
+        // the module lookup errors have been emitted. While the
+        // ClangDiagnosticConsumer takes care of filtering out the diagnostics
+        // from the output and from being handled by Swift's DiagnosticEngine,
+        // we must ensure to also reset Clang's DiagnosticEngine because its
+        // state is queried in later stages of compilation and errors emitted on
+        // "module_not_found" should not be counted. Use a soft reset that only
+        // clear the errors but not reset the states.
+        // FIXME: We must instead allow for module loading in Clang to fail
+        // without needing to emit a diagnostic.
+        if (Engine && Consumer->CurrentImportNotFound &&
+            DiagEngineClearPriorToLookup)
+          Engine->Reset(/*soft=*/true);
         Consumer->CurrentImport = nullptr;
         Consumer->CurrentImportNotFound = false;
       }