[lldb][NFC] Simplify ClangASTContext::GetTranslationUnitDecl

These two functions are just calling their equivalent function
in ASTContext and implicitly convert the result to a
DeclContext* (a parent class of TranslationUnitDecl). This leads
to the absurd situation that we had to cast the result of
GetTranslationUnitDecl to a TranslationUnitDecl*. The only reason
we did this implicit conversion to the parent class
was that the void* conversion for the CompilerDeclContext constructor
was sound (which otherwise would receive a Decl* pointer when
called with a TranslationUnitDecl*).

Now that the CompilerDeclContext constructor is type safe we can
properly implement these functions by actually returning the
right type. Also deletes the static inconvenience method that was
not used anywhere.