d: Fix alias protection being ignored if used before declaration.

d: Fix alias protection being ignored if used before declaration.

Fixes a symbol resolver bug where a private alias becomes public if used
before its declaration.

gcc/d/ChangeLog:

2020-10-12  Iain Buclaw  <ibuclaw@gdcproject.org>

	* dmd/declaration.c (AliasDeclaration::aliasSemantic): Apply storage
	class and protection attributes.

gcc/testsuite/ChangeLog:

2020-10-12  Iain Buclaw  <ibuclaw@gdcproject.org>

	* gdc.test/fail_compilation/fail21001.d: New test.
	* gdc.test/fail_compilation/imports/fail21001b.d: New test.
	* gdc.test/fail_compilation/imports/issue21295ast_node.d: New test.
	* gdc.test/fail_compilation/imports/issue21295astcodegen.d: New test.
	* gdc.test/fail_compilation/imports/issue21295dtemplate.d: New test.
	* gdc.test/fail_compilation/imports/issue21295visitor.d: New test.
	* gdc.test/fail_compilation/issue21295.d: New test.

(cherry picked from commit c3249210b0108b5fc2dc403e558ab63882293be4)