[Frontend] Flip default of CreateInvocationOptions::ProbePrecompiled to false

This is generally a better default for tools other than the compiler, which
shouldn't assume a PCH file on disk is something they can consume.

Preserve the old behavior in places associated with libclang/c-index-test
(including ASTUnit) as there are tests relying on it and most important
consumers are out-of-tree. It's unclear whether the tests are specifically
trying to test this functionality, and what the downstream implications of
removing it are. Hopefully someone more familiar can clean this up in future.

Differential Revision: https://reviews.llvm.org/D125149

clang/include/clang/Frontend/Utils.h

@@ -206,7 +206,7 @@ struct CreateInvocationOptions {
   /// This is used to replace -include with -include-pch in the cc1 args.
   /// FIXME: ProbePrecompiled=true is a poor, historical default.
   /// It misbehaves if the PCH file is from GCC, has the wrong version, etc.
-  bool ProbePrecompiled = true;
+  bool ProbePrecompiled = false;
   /// If set, the target is populated with the cc1 args produced by the driver.
   /// This may be populated even if createInvocation returns nullptr.
   std::vector<std::string> *CC1Args = nullptr;

clang/lib/Frontend/ASTUnit.cpp

@@ -1732,6 +1732,7 @@ ASTUnit *ASTUnit::LoadFromCommandLine(
     CreateInvocationOptions CIOpts;
     CIOpts.VFS = VFS;
     CIOpts.Diags = Diags;
+    CIOpts.ProbePrecompiled = true; // FIXME: historical default. Needed?
     CI = createInvocation(llvm::makeArrayRef(ArgBegin, ArgEnd),
                           std::move(CIOpts));
     if (!CI)

clang/tools/c-index-test/core_main.cpp

@@ -223,6 +223,7 @@ static bool printSourceSymbols(const char *Executable,
     Diags(CompilerInstance::createDiagnostics(new DiagnosticOptions));
   CreateInvocationOptions CIOpts;
   CIOpts.Diags = Diags;
+  CIOpts.ProbePrecompiled = true; // FIXME: historical default. Needed?
   auto CInvok = createInvocation(ArgsWithProgName, std::move(CIOpts));
   if (!CInvok)
     return true;

clang/tools/libclang/Indexing.cpp

@@ -510,6 +510,7 @@ static CXErrorCode clang_indexSourceFile_Impl(
 
   CreateInvocationOptions CIOpts;
   CIOpts.Diags = Diags;
+  CIOpts.ProbePrecompiled = true; // FIXME: historical default. Needed?
   std::shared_ptr<CompilerInvocation> CInvok =
       createInvocation(*Args, std::move(CIOpts));
 

clang/unittests/Frontend/UtilsTest.cpp

@@ -48,20 +48,20 @@ TEST(BuildCompilerInvocationTest, ProbePrecompiled) {
   llvm::IntrusiveRefCntPtr<DiagnosticsEngine> CommandLineDiagsEngine =
       clang::CompilerInstance::createDiagnostics(new DiagnosticOptions, &D,
                                                  false);
-  // Default: ProbePrecompiled is true.
+  // Default: ProbePrecompiled=false
   CreateInvocationOptions CIOpts;
   CIOpts.Diags = CommandLineDiagsEngine;
   CIOpts.VFS = FS;
   std::unique_ptr<CompilerInvocation> CI = createInvocation(Args, CIOpts);
   ASSERT_TRUE(CI);
-  EXPECT_THAT(CI->getPreprocessorOpts().Includes, ElementsAre());
-  EXPECT_EQ(CI->getPreprocessorOpts().ImplicitPCHInclude, "foo.h.pch");
-
-  CIOpts.ProbePrecompiled = false;
-  CI = createInvocation(Args, CIOpts);
-  ASSERT_TRUE(CI);
   EXPECT_THAT(CI->getPreprocessorOpts().Includes, ElementsAre("foo.h"));
   EXPECT_EQ(CI->getPreprocessorOpts().ImplicitPCHInclude, "");
+
+  CIOpts.ProbePrecompiled = true;
+  CI = createInvocation(Args, CIOpts);
+  ASSERT_TRUE(CI);
+  EXPECT_THAT(CI->getPreprocessorOpts().Includes, ElementsAre());
+  EXPECT_EQ(CI->getPreprocessorOpts().ImplicitPCHInclude, "foo.h.pch");
 }
 
 } // namespace