Fix cxx_variable_templates feature check and documentation

This C++ feature has been marked complete since r191549, but the documentation
claimed it wasn't supported at all and the extension check misreported it as
being available in C.

No regression test; this was a short-lived typo.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199292 91177308-0d34-0410-b5e6-96231b3b80d8

docs/LanguageExtensions.rst

diff --git a/docs/LanguageExtensions.rst b/docs/LanguageExtensions.rst
index 682cc98..24ef855 100644
--- a/docs/LanguageExtensions.rst
+++ b/docs/LanguageExtensions.rst
@@ -873,7 +873,6 @@
 Use ``__has_feature(cxx_variable_templates)`` or
 ``__has_extension(cxx_variable_templates)`` to determine if support for
 templated variable declarations is enabled.
-Clang does not yet support this feature.
 
 C11
 ---

lib/Lex/PPMacroExpansion.cpp

diff --git a/lib/Lex/PPMacroExpansion.cpp b/lib/Lex/PPMacroExpansion.cpp
index c6559b4..721760a 100644
--- a/lib/Lex/PPMacroExpansion.cpp
+++ b/lib/Lex/PPMacroExpansion.cpp
@@ -1039,7 +1039,7 @@
            // C++1y features supported by other languages as extensions.
            .Case("cxx_binary_literals", true)
            .Case("cxx_init_captures", LangOpts.CPlusPlus11)
-           .Case("cxx_variable_templates", true)
+           .Case("cxx_variable_templates", LangOpts.CPlusPlus)
            .Default(false);
 }