[clangd] Drop includes from disabled PP regions in preamble patch

[kadircet]

Summary

In rest of the clangd functionality we treat these includes as
non-existent. Do so under preamble patching.

Depends on D143197

clang-tools-extra/clangd/Preamble.cpp

if (IncludesChanged && PatchType == PatchType::All) {
     for (const auto &Inc : BaselineScan->Includes)
       ExistingIncludes.try_emplace({Inc.Directive, Inc.Written});
     // Calculate extra includes that needs to be inserted.
     for (auto &Inc : ModifiedScan->Includes) {
       auto It = ExistingIncludes.find({Inc.Directive, Inc.Written});
       // Include already present in the baseline preamble. Set resolved path and
       // put into preamble includes.
       if (It != ExistingIncludes.end()) {
+        if (It->second) {
+          // If this header is included in an active region of the baseline
+          // preamble, preserve it.
           auto &PatchedInc = PP.PreambleIncludes.emplace_back();
-        // Copy everything from existing include, apart from the location, when
-        // it's coming from baseline preamble.
-        if (It->second)
+          // Copy everything from existing include, apart from the location,
+          // when it's coming from baseline preamble.
           PatchedInc = *It->second;
           PatchedInc.HashLine = Inc.HashLine;
           PatchedInc.HashOffset = Inc.HashOffset;
+        }
         continue;
       }
       // Include is new in the modified preamble. Inject it into the patch and
       // use #line to set the presumed location to where it is spelled.
       auto LineCol = offsetToClangLineColumn(Modified.Contents, Inc.HashOffset);
       Patch << llvm::formatv("#line {0}\n", LineCol.first);
       Patch << llvm::formatv(
           "#{0} {1}\n", spellingForIncDirective(Inc.Directive), Inc.Written);

clang-tools-extra/clangd/unittests/PreambleTests.cpp

 }
 
 TEST(PreamblePatchTest, PatchesPreambleIncludes) {
   MockFS FS;
   IgnoreDiagnostics Diags;
   auto TU = TestTU::withCode(R"cpp(
     #include "a.h" // IWYU pragma: keep
     #include "c.h"
+    #ifdef FOO
+    #include "d.h"
+    #endif
   )cpp");
   TU.AdditionalFiles["a.h"] = "#include \"b.h\"";
   TU.AdditionalFiles["b.h"] = "";
   TU.AdditionalFiles["c.h"] = "";
   auto PI = TU.inputs(FS);
   auto BaselinePreamble = buildPreamble(
       TU.Filename, *buildCompilerInvocation(PI, Diags), PI, true, nullptr);
   // We drop c.h from modified and add a new header. Since the latter is patched
-  // we should only get a.h in preamble includes.
+  // we should only get a.h in preamble includes. d.h shouldn't be part of the
+  // preamble, as it's coming from a disabled region.
   TU.Code = R"cpp(
     #include "a.h"
     #include "b.h"
+    #ifdef FOO
+    #include "d.h"
+    #endif
   )cpp";
   auto PP = PreamblePatch::createFullPatch(testPath(TU.Filename), TU.inputs(FS),
                                            *BaselinePreamble);
   // Only a.h should exists in the preamble, as c.h has been dropped and b.h was
   // newly introduced.
   EXPECT_THAT(
       PP.preambleIncludes(),
       ElementsAre(AllOf(