[Clang] Unify interface for accessing template arguments as written for class/variable template specializations (#81642)

Our current method of storing the template arguments as written for
`(Class/Var)Template(Partial)SpecializationDecl` suffers from a number
of flaws:
- We use `TypeSourceInfo` to store `TemplateArgumentLocs` for class
template/variable template partial/explicit specializations. For
variable template specializations, this is a rather unintuitive hack (as
we store a non-type specialization as a type). Moreover, we don't ever
*need* the type as written -- in almost all cases, we only want the
template arguments (e.g. in tooling use-cases).
- The template arguments as written are stored in a number of redundant
data members. For example, `(Class/Var)TemplatePartialSpecialization`
have their own `ArgsAsWritten` member that stores an
`ASTTemplateArgumentListInfo` (the template arguments).
`VarTemplateSpecializationDecl` has yet _another_ redundant member
"`TemplateArgsInfo`" that also stores an `ASTTemplateArgumentListInfo`.

This patch eliminates all
`(Class/Var)Template(Partial)SpecializationDecl` members which store the
template arguments as written, and turns the `ExplicitInfo` member into
a `llvm::PointerUnion<const ASTTemplateArgumentListInfo*,
ExplicitInstantiationInfo*>` (to avoid unnecessary allocations when the
declaration isn't an explicit instantiation). The template arguments as
written are now accessed via `getTemplateArgsWritten` in all cases.

The "most breaking" change is to AST Matchers, insofar that `hasTypeLoc`
will no longer match class template specializations (since they no
longer store the type as written).

1 files changed, 2 insertions, 1 deletions

diff --git a/clang/lib/Tooling/Syntax/BuildTree.cpp b/clang/lib/Tooling/Syntax/BuildTree.cpp
index cd02619..3e50d67f 100644
--- a/clang/lib/Tooling/Syntax/BuildTree.cpp
+++ b/clang/lib/Tooling/Syntax/BuildTree.cpp
@@ -735,7 +735,8 @@ public:
     auto *Declaration =
         cast<syntax::SimpleDeclaration>(handleFreeStandingTagDecl(C));
     foldExplicitTemplateInstantiation(
-        Builder.getTemplateRange(C), Builder.findToken(C->getExternLoc()),
+        Builder.getTemplateRange(C),
+        Builder.findToken(C->getExternKeywordLoc()),
         Builder.findToken(C->getTemplateKeywordLoc()), Declaration, C);
     return true;
   }