[Ada] Accept concatentation arguments to pragma Annotate

[Ada] Accept concatentation arguments to pragma Annotate

In cases where pragma Annotate accepts a string literal as an argument,
we now also accept a concatenation of string literals.

2019-09-19  Steve Baird  <baird@adacore.com>

gcc/ada/

	* sem_prag.adb (Preferred_String_Type): A new function. Given an
	expression, determines whether the preference rules defined for
	the third-and-later arguments of pragma Annotate suffice to
	determine the type of the expression. If so, then the preferred
	type is returned; if not then Empty is returned. Handles
	concatenations.
	(Analyze_Pragma): Replace previous code, which dealt only with
	string literals, with calls to the new Preferred_String_Type
	function, which also handles concatenations.
	* doc/gnat_rm/implementation_defined_pragmas.rst: Update
	documentation for pragma Annotate.
	* gnat_rm.texi: Regenerate.

gcc/testsuite/

	* gnat.dg/annotation1.adb: New testcase.

From-SVN: r275957