[Sema] Mark target of __attribute__((alias("target"))) used for C

Summary:
Prevents -Wunneeded-internal-delcaration warnings when the target has no
other references. This occurs frequently in device drivers in the Linux
kernel.

Sema would need to invoke the demangler on the target, since in C++ the
target name is mangled:

int f() { return 42; }
int g() __attribute__((alias("_Z1fv")));

Sema does not have the ability to demangle names at this time.

https://bugs.llvm.org/show_bug.cgi?id=39088
https://github.com/ClangBuiltLinux/linux/issues/232

Reviewers: rsmith, rjmccall

Reviewed By: rsmith

Subscribers: erik.pilkington, cfe-commits, pirama, srhines

Differential Revision: https://reviews.llvm.org/D54188

llvm-svn: 350776