[clang] Add the exclude_from_explicit_instantiation attribute

Summary:
This attribute allows excluding a member of a class template from being part
of an explicit template instantiation of that class template. This also makes
sure that code using such a member will not take for granted that an external
instantiation exists in another translation unit. The attribute was discussed
on cfe-dev at [1] and is primarily motivated by the removal of always_inline
in libc++ to control what's part of the ABI (see links in [1]).

[1]: http://lists.llvm.org/pipermail/cfe-dev/2018-August/059024.html

rdar://problem/43428125

Reviewers: rsmith

Subscribers: dexonsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D51789

llvm-svn: 343790
diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
index a437f15..c0fb335 100644
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -6512,6 +6512,9 @@
   case ParsedAttr::AT_InternalLinkage:
     handleInternalLinkageAttr(S, D, AL);
     break;
+  case ParsedAttr::AT_ExcludeFromExplicitInstantiation:
+    handleSimpleAttribute<ExcludeFromExplicitInstantiationAttr>(S, D, AL);
+    break;
   case ParsedAttr::AT_LTOVisibilityPublic:
     handleSimpleAttribute<LTOVisibilityPublicAttr>(S, D, AL);
     break;