[clangd] Disable delayed template parsing in the main file

[sammccall]

Summary

This is on by default in windows and breaks most features in template bodies.
We'd already disabled it in code completion, now disable it for building ASTs.

Potential regressions:

• we may give spurious errors where files with templates relying on delayed parsing are directly opened
• we may misparse such template bodies that are instantiated (and therefore *were* previously parsed)

Still *probably* a win overall. Avoiding the regressions entirely would be
substantial work and we don't have plans for it now.

Fixes https://github.com/clangd/clangd/issues/302 (again)

[kadircet]

thanks, LGTM!

clang-tools-extra/clangd/ParsedAST.cpp

ParsedAST::build(llvm::StringRef Version,
   // to leak memory in clangd.
   CI->getFrontendOpts().DisableFree = false;
   const PrecompiledPreamble *PreamblePCH =
       Preamble ? &Preamble->Preamble : nullptr;
 
   // Recovery expression currently only works for C++.
   if (CI->getLangOpts()->CPlusPlus)
     CI->getLangOpts()->RecoveryAST = Opts.BuildRecoveryAST;
+  // This is on-by-default in windows to allow parsing SDK headers, but it
+  // breaks many features. Disable it for the main-file (not preamble).
+  CI->getLangOpts()->DelayedTemplateParsing = false;
 
   StoreDiags ASTDiags;
   std::string Content = std::string(Buffer->getBuffer());
   std::string Filename =
       std::string(Buffer->getBufferIdentifier()); // Absolute.
 
   auto Clang = prepareCompilerInstance(std::move(CI), PreamblePCH,
                                        std::move(Buffer), VFS, ASTDiags);

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

EXPECT_THAT(
                         AllOf(DeclNamed("V"), WithTemplateArgs("<bool>")),
                         AllOf(DeclNamed("foo"), WithTemplateArgs("")),
                         AllOf(DeclNamed("i"), WithTemplateArgs("")),
                         AllOf(DeclNamed("d"), WithTemplateArgs("")),
                         AllOf(DeclNamed("foo"), WithTemplateArgs("<T *>")),
                         AllOf(DeclNamed("foo"), WithTemplateArgs("<bool>"))}));
 }
 
+TEST(ParsedASTTest, IgnoresDelayedTemplateParsing) {
+  auto TU = TestTU::withCode(R"cpp(
+    template <typename T> void xxx() {
+      int yyy = 0;
+    }
+  )cpp");
+  TU.ExtraArgs.push_back("-fdelayed-template-parsing");
+  auto AST = TU.build();
+  EXPECT_EQ(Decl::Var, findUnqualifiedDecl(AST, "yyy").getKind());
+}
+
 TEST(ParsedASTTest, TokensAfterPreamble) {
   TestTU TU;
   TU.AdditionalFiles["foo.h"] = R"(
     int foo();
   )";
   TU.Code = R"cpp(
       #include "foo.h"
       first_token;