[clang][TemplateBase] Add IsDefaulted bit to TemplateArgument

**Summary**

This patch adds a `IsDefaulted` field to `clang::TemplateArgument`.

To prevent memory footprint increase we still 1 bit from `ArgKind`.

**Changes**

1. `getIsDefaulted`/`setIsDefaulted` to allow clients to communicate
   an argument's defaulted-ness to the TypePrinter
2. The `TemplateArgument` properties description had to be changed
   to make sure we correctly mark the defaulted-ness of arguments
   that came from a deserialized AST (caught by the HLSL test-suite)
3. The `TemplateArgument` constructors now accept a `IsDefaulted`
   parameter to simplify construction from the tablegen description.
   Though if people don't want to clutter the constructors we can
   instead call `setIsDefaulted` from tablegen
4. When `clang::Sema` checks the template arguments against template
   parameters we now call `setIsDefaulted`. This makes sure that
   whenever a specialization decl gets constructed, the defaulted-ness
   of the associated `TemplateArgument`s has already been deduced.
   This preserves the immutability of `TemplateArgumentList`s

**Background**

In LLDB we construct ASTs from debug-info and hand it to clang
to perform actions such as printing/formatting a typenames.
Some debug formats, specifically DWARF, may only encode information
about class template instantiations, losing the structure of the generic
class definition. However, the `clang::TypePrinter` needs a properly
constructed `ClassTemplateDecl` with generic default argument decls
to be able to deduce whether a `ClassTemplateSpecializationDecl` was
instantiatiated with `TemplateArgument`s that correspond to the
defaults. LLDB does know whether a particular template argument was
defaulted, but can't currently tell clang about it.

This patch allows LLDB to set the defaulted-ness of a `TemplateArgument`
and thus benefit more from `clang::TypePrinter`.

See discussion in https://reviews.llvm.org/D140423

**Testing**

* Added unit-test
* LLDB/clang/llvm test-suite passes

Differential Revision: https://reviews.llvm.org/D141826

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