[Tooling] Add -x flags when inferring compile commands for files with no/invalid extension.

[sammccall]

Summary

We treat them as headers, as the motivating case is C++ standard library.

lib/Tooling/InterpolatingCompilationDatabase.cpp

struct TransferableCommand {
 
   // Produce a CompileCommand for \p filename, based on this one.
   CompileCommand transferTo(StringRef Filename) const {
     CompileCommand Result = Cmd;
     Result.Filename = Filename;
     bool TypeCertain;
     auto TargetType = guessType(Filename, &TypeCertain);
     // If the filename doesn't determine the language (.h), transfer with -x.
-    if (TargetType != types::TY_INVALID && !TypeCertain && Type) {
-      TargetType = types::onlyPrecompileType(TargetType) // header?
-                       ? types::lookupHeaderTypeForSourceType(*Type)
-                       : *Type;
+    if ((!TargetType || !TypeCertain) && Type) {
+      // Use *Type, or its header variant if the file is a header.
+      // Treat no/invalid extension as header (e.g. C++ standard library).
+      TargetType =
+          (!TargetType || types::onlyPrecompileType(TargetType)) // header?
       if (ClangCLMode) {
         const StringRef Flag = toCLFlag(TargetType);
         if (!Flag.empty())
           Result.CommandLine.push_back(Flag);
       } else {
         Result.CommandLine.push_back("-x");
         Result.CommandLine.push_back(types::getTypeName(TargetType));
       }

unittests/Tooling/CompilationDatabaseTest.cpp

 TEST_F(InterpolateTest, Language) {
   add("dir/foo.cpp", "-std=c++17");
   add("dir/bar.c", "");
   add("dir/baz.cee", "-x c");
 
   // .h is ambiguous, so we add explicit language flags
   EXPECT_EQ(getCommand("foo.h"),
             "clang -D dir/foo.cpp -x c++-header -std=c++17");
+  // Same thing if we have no extension. (again, we treat as header).
+  EXPECT_EQ(getCommand("foo"), "clang -D dir/foo.cpp -x c++-header -std=c++17");
+  // and invalid extensions.
+  EXPECT_EQ(getCommand("foo.cce"),
+            "clang -D dir/foo.cpp -x c++-header -std=c++17");
   // and don't add -x if the inferred language is correct.
   EXPECT_EQ(getCommand("foo.hpp"), "clang -D dir/foo.cpp -std=c++17");
   // respect -x if it's already there.
   EXPECT_EQ(getCommand("baz.h"), "clang -D dir/baz.cee -x c-header");
   // prefer a worse match with the right extension.
   EXPECT_EQ(getCommand("foo.c"), "clang -D dir/bar.c");
-  // make sure we don't crash on queries with invalid extensions.
-  EXPECT_EQ(getCommand("foo.cce"), "clang -D dir/foo.cpp");
   Entries.erase(path(StringRef("dir/bar.c")));
   // Now we transfer across languages, so drop -std too.
   EXPECT_EQ(getCommand("foo.c"), "clang -D dir/foo.cpp");
 }
 
 TEST_F(InterpolateTest, Strip) {
   add("dir/foo.cpp", "-o foo.o -Wall");
   // the -o option and the input file are removed, but -Wall is preserved.