Export TemplateArgumentMatcher so clients defining custom matchers don't need to...

Export TemplateArgumentMatcher so clients defining custom matchers don't need to use the internal namespace

This change adds another export, `using TemplateArgumentMatcher = internal::Matcher<TemplateArgument>;`, to the collection of exports that put instantiations of the `clang::ast_matchers::internal::Matcher` into the `clang::ast_matchers` namespace. This makes it possible to define custom TemplateArgument matchers without reaching into the `internal` namespace.

Reviewed By: klimek

Differential Revision: https://reviews.llvm.org/D89920