[clangd] Add C++20 concepts support to TargetFinder

[merge_guards_bot]

Unit tests: pass. 62082 tests passed, 0 failed and 784 were skipped.

clang-tidy: unknown.

clang-format: pass.

Build artifacts: diff.json, clang-format.patch, CMakeCache.txt, console-log.txt, test-results.xml

clang-tools-extra/clangd/FindTarget.cpp

 #include "clang/AST/ASTTypeTraits.h"
 #include "clang/AST/Decl.h"
 #include "clang/AST/DeclCXX.h"
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/DeclVisitor.h"
 #include "clang/AST/DeclarationName.h"
 #include "clang/AST/Expr.h"
 #include "clang/AST/ExprCXX.h"
+#include "clang/AST/ExprConcepts.h"
 #include "clang/AST/ExprObjC.h"
 #include "clang/AST/NestedNameSpecifier.h"
 #include "clang/AST/PrettyPrinter.h"
 #include "clang/AST/RecursiveASTVisitor.h"
 #include "clang/AST/StmtVisitor.h"
 #include "clang/AST/TemplateBase.h"
 #include "clang/AST/Type.h"
 #include "clang/AST/TypeLoc.h"
void add(const Stmt *S, RelSet Flags) {
     struct Visitor : public ConstStmtVisitor<Visitor> {
       TargetFinder &Outer;
       RelSet Flags;
       Visitor(TargetFinder &Outer, RelSet Flags) : Outer(Outer), Flags(Flags) {}
 
       void VisitCallExpr(const CallExpr *CE) {
         Outer.add(CE->getCalleeDecl(), Flags);
       }
+      void VisitConceptSpecializationExpr(const ConceptSpecializationExpr *E) {
+        Outer.add(E->getNamedConcept(), Flags);
+      }
       void VisitDeclRefExpr(const DeclRefExpr *DRE) {
         const Decl *D = DRE->getDecl();
         // UsingShadowDecl allows us to record the UsingDecl.
         // getFoundDecl() returns the wrong thing in other cases (templates).
         if (auto *USD = llvm::dyn_cast<UsingShadowDecl>(DRE->getFoundDecl()))
           D = USD;
         Outer.add(D, Flags);
       }

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

void foo() {
       [[Test]] a(5);
     }
   )cpp";
   Flags.push_back("-std=c++17");
   EXPECT_DECLS("DeducedTemplateSpecializationTypeLoc",
                {"struct Test", Rel::TemplatePattern});
 }
 
+TEST_F(TargetDeclTest, Concept) {
+  Code = R"cpp(
+    template <typename T>
+    concept Fooable = requires (T t) { t.foo(); };
+
+    template <typename T> requires [[Fooable]]<T>
+    void bar(T t) {
+      t.foo();
+    }
+  )cpp";
+  Flags.push_back("-std=c++2a");
+  EXPECT_DECLS(
+      "ConceptSpecializationExpr",
+      // FIXME: Should we truncate the pretty-printed form of a concept decl
+      // somewhere?
+      {"template <typename T> concept Fooable = requires (T t) { t.foo(); };"});
+}
+
 TEST_F(TargetDeclTest, FunctionTemplate) {
   Code = R"cpp(
     // Implicit specialization.
     template<typename T> bool foo(T) { return false; };
     bool x = [[foo]](42);
   )cpp";
   EXPECT_DECLS("DeclRefExpr",
                {"template<> bool foo<int>(int)", Rel::TemplateInstantiation},