Switch the ValueStore-related templates to use explicit instantiation (#7116)

As part of this, move functions that seem reasonable to make out-of-line
to a separate `_impl.h` header file that is only included where the
explicit instantiation _definition_ is provided.

By using explicit instantiation we can make these templates behave more
like non-template classes in terms of supporting out-of-line definitions
that don't need to be compiled by every translation unit. The set of
eventual instantiations here is fundamentally known, and there tend to
be headers that define a canonical "leaf" type where it makes sense to
trigger the explicit instantiation.

Where we already had a `.cpp` file to put the explicit instantiation
definition, use it. But in some places we didn't have such a `.cpp` file
so this PR adds those.

This also requires that we have precise constraints on APIs that _can't_
be instantiated for specific argument types, as now we don't do this
lazily.

Combined, this appears to reduce the sum of object file sizes in the
`check` directory by almost 40% (122mb -> 74mb) in my measurement.

My actual goal was to improve compile times, but so far I don't have a
great methodology for measuring these... But the object file size
reduction seems to confirm this is a net win and likely represents a
non-trivial improvement in compile time.

Assisted-by: Antigravity with Gemini

toolchain/parse/tree.cpp

@@ -8,6 +8,7 @@
 #include "common/error.h"
 #include "llvm/ADT/Sequence.h"
 #include "llvm/ADT/SmallVector.h"
+#include "toolchain/base/value_store_impl.h"
 #include "toolchain/lex/tokenized_buffer.h"
 #include "toolchain/parse/node_kind.h"
 #include "toolchain/parse/tree_and_subtrees.h"
@@ -87,3 +88,8 @@ auto Tree::PostorderIterator::Print(llvm::raw_ostream& output) const -> void {
 }
 
 }  // namespace Carbon::Parse
+
+namespace Carbon {
+template class ValueStore<Parse::DeferredDefinitionIndex,
+                          Parse::DeferredDefinition>;
+}  // namespace Carbon