[Clang][CodeGen] set alias linkage on QualType

Summary:
It seems that CodeGen was always using ExternalLinkage when emitting a
GlobalDecl with __attribute__((alias)). This leads to symbol
redefinitions (ODR) that cause failures at link time for static aliases.
This is readily attempting to link an ARM (32b) allyesconfig Linux
kernel built with Clang.

Reported-by: nathanchance
Suggested-by: ihalip
Link: https://bugs.llvm.org/show_bug.cgi?id=42377
Link: https://github.com/ClangBuiltLinux/linux/issues/631

Reviewers: rsmith, aaron.ballman, erichkeane

Reviewed By: aaron.ballman

Subscribers: javed.absar, kristof.beyls, cfe-commits, srhines, ihalip, nathanchance

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66492

llvm-svn: 369705