Debug Info: Add a file: field to DIImportedEntity.

DIImportedEntity has a line number, but not a file field. To determine
the decl_line/decl_file we combine the line number from the
DIImportedEntity with the file from the DIImportedEntity's scope. This
does not work correctly when the parent scope is a DINamespace or a
DIModule, both of which do not have a source file.

This patch adds a file field to DIImportedEntity to unambiguously
identify the source location of the using/import declaration.  Most
testcase updates are mechanical, the interesting one is the removal of
the FIXME in test/DebugInfo/Generic/namespace.ll.

This fixes PR33822. See https://bugs.llvm.org/show_bug.cgi?id=33822
for more context.

<rdar://problem/33357889>
https://bugs.llvm.org/show_bug.cgi?id=33822

Differential Revision: https://reviews.llvm.org/D35583

llvm-svn: 308398