[Concepts] Add (disabled) __has_feature/extension; NFC

[hubert.reinterpretcast]

Summary

This adds a commented-out line which would otherwise enable
__has_feature and __has_extension queries for the Concepts TS and an
equally commented-out section on the Clang Language Extensions page.

docs/LanguageExtensions.rst

 
 C++1y variable templates
 ^^^^^^^^^^^^^^^^^^^^^^^^
 
 Use ``__has_feature(cxx_variable_templates)`` or
 ``__has_extension(cxx_variable_templates)`` to determine if support for
 templated variable declarations is enabled.
 
+..
+  C++ Technical Specifications
+  ----------------------------
+..
+  The features listed below are part of the various C++ Technical Specifications
+  or working papers, preliminary drafts, drafts, etc. thereof. These features
+  may be enabled through the option associated with their respective TS when
+  compiling C++ code.
+..
+  C++ Extensions for Concepts
+  ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+..
+  Use ``__has_feature(cxx_experimental_concepts)`` or
+  ``__has_extension(cxx_experimental_concepts)`` to determine if support for
+  the C++ extensions for concepts is enabled.
+
 C11
 ---
 
 The features listed below are part of the C11 standard.  As a result, all these
 features are enabled with the ``-std=c11`` or ``-std=gnu11`` option when
 compiling C code.  Additionally, because these features are all
 backward-compatible, they are available as extensions in all language modes.
 

lib/Lex/PPMacroExpansion.cpp

return llvm::StringSwitch<bool>(Feature)
       .Case("cxx_decltype_auto", LangOpts.CPlusPlus14)
       .Case("cxx_generic_lambdas", LangOpts.CPlusPlus14)
       .Case("cxx_init_captures", LangOpts.CPlusPlus14)
       .Case("cxx_relaxed_constexpr", LangOpts.CPlusPlus14)
       .Case("cxx_return_type_deduction", LangOpts.CPlusPlus14)
       .Case("cxx_variable_templates", LangOpts.CPlusPlus14)
       // C++ TSes
       //.Case("cxx_runtime_arrays", LangOpts.CPlusPlusTSArrays)
-      //.Case("cxx_concepts", LangOpts.CPlusPlusTSConcepts)
+      //.Case("cxx_experimental_concepts", LangOpts.ConceptsTS)
       // FIXME: Should this be __has_feature or __has_extension?
       //.Case("raw_invocation_type", LangOpts.CPlusPlus)
       // Type traits
       .Case("has_nothrow_assign", LangOpts.CPlusPlus)
       .Case("has_nothrow_copy", LangOpts.CPlusPlus)
       .Case("has_nothrow_constructor", LangOpts.CPlusPlus)
       .Case("has_trivial_assign", LangOpts.CPlusPlus)
       .Case("has_trivial_copy", LangOpts.CPlusPlus)

test/Lexer/has_extension_cxx.cpp

 // RUN: %clang_cc1 -std=c++98 -E %s -o - | FileCheck %s
 // RUN: %clang_cc1 -std=c++11 -E %s -o - | FileCheck %s --check-prefix=CHECK11
+// RUN: %clang_cc1 -std=c++14 -E %s -o - | FileCheck %s --check-prefix=CHECK14
+// RUN: %clang_cc1 -std=c++1z -E %s -o - | FileCheck %s --check-prefix=CHECK1Z
 
 // CHECK: c_static_assert
 #if __has_extension(c_static_assert)
 int c_static_assert();
 #endif
 
 // CHECK: c_generic_selections
 #if __has_extension(c_generic_selections)

 int has_variable_templates();
 #endif
 
 // CHECK-NOT: has_init_captures
 // CHECK11: has_init_captures
 #if __has_extension(cxx_init_captures)
 int has_init_captures();
 #endif
+
+// CHECK-NOT: has_experimental_concepts
+// CHECK11-NOT: has_experimental_concepts
+// CHECK14-NOT: has_experimental_concepts
+// CHECK1Z-NOT: has_experimental_concepts
+#if __has_extension(cxx_experimental_concepts)
+int has_experimental_concepts();
+#endif

test/Lexer/has_feature_cxx0x.cpp

 #else
 int no_generic_lambdas();
 #endif
 
 // CHECK-1Z: has_generic_lambdas
 // CHECK-14: has_generic_lambdas
 // CHECK-11: no_generic_lambdas
 // CHECK-NO-11: no_generic_lambdas
+
+#if __has_feature(cxx_experimental_concepts)
+int has_experimental_concepts();
+#else
+int no_experimental_concepts();
+#endif
+
+// CHECK-1Z: no_experimental_concepts
+// CHECK-14: no_experimental_concepts
+// CHECK-11: no_experimental_concepts
+// CHECK-NO-11: no_experimental_concepts