[pch] Honour -fallow-pch-with-compiler-errors for overall compilation status

[benlangmuir]

Summary

Previously we would emit a PCH with errors, but fail the overall
compilation. If run using the driver, that would result in removing the
just-produced PCH. Instead, we should have the compilation result match
whether we were able to emit the PCH.

rdar://61110294

clang/lib/Serialization/GeneratePCH.cpp

if (PP.getLangOpts().isCompilingModule()) {
     Module = PP.getHeaderSearchInfo().lookupModule(
         PP.getLangOpts().CurrentModule, /*AllowSearch*/ false);
     if (!Module) {
       assert(hasErrors && "emitting module but current module doesn't exist");
       return;
     }
   }
 
+  // Errors that do not prevent the PCH from being written should not cause the
+  // overall compilation to fail either.
+  if (AllowASTWithErrors)
+    PP.getDiagnostics().getClient()->clear();
+
   // Emit the PCH file to the Buffer.
   assert(SemaPtr && "No Sema?");
   Buffer->Signature =
       Writer.WriteAST(*SemaPtr, OutputFile, Module, isysroot,
                       // For serialization we are lenient if the errors were
                       // only warn-as-error kind.
                       PP.getDiagnostics().hasUncompilableErrorOccurred(),
                       ShouldCacheASTInMemory);

clang/test/Index/pch-with-errors.c

 // CHECK-INDEX: [indexDeclaration]: kind: function | name: foo
 // CHECK-INDEX: [indexEntityReference]: kind: function | name: erroneous
 
 // RUN: not %clang -fsyntax-only %s -include %t.h 2>&1 | FileCheck -check-prefix=PCH-ERR %s
 // PCH-ERR: error: PCH file contains compiler errors
 
 // RUN: not c-index-test -write-pch %t.pch foobar.c 2>&1 | FileCheck -check-prefix=NONEXISTENT %s
 // NONEXISTENT: Unable to load translation unit
+
+// RUN: %clang -x c-header %s -o %t-clang.h.pch -Xclang -detailed-preprocessing-record -Xclang -fallow-pch-with-compiler-errors
+// RUN: c-index-test -index-file %s -include %t-clang.h -Xclang -detailed-preprocessing-record | FileCheck -check-prefix=CHECK-INDEX %s