Adding hasDeclaration overload for TemplateSpecializationType

TemplateSpecializationType doesn't quite have getDecl(). Need to go
through TemplateName to get a TemplateDecl.

Added test cases for the hasDeclaration() overload for
TemplateSpecializationType. Also introduced the type matcher
templateSpecializationType() used by the new hasDeclaration() test case.

Updated LibASTMatchersReference.

Reviewers: klimek