[Clang][CodeGen] support alias attribute w/ gnu_inline
r369705 did not consider the addition of gnu_inline on function
declarations of alias attributed functions. This resulted in a reported
regression in the clang-9-rc4 release from the Zig developers building
glibc, which was observable as a failed assertion:
Assertion `(doesThisDeclarationHaveABody() || willHaveBody()) && "Must
be a function definition"' failed.
Alias function declarations do not have bodies, so allow us to proceed
if we have the alias function attribute but no body/definition, and add
a test case. The emitted symbols and their linkage matches GCC for the
added test case.
Reviewers: aaron.ballman, rsmith, erichkeane, andrewrk
Reviewed By: andrewrk
Subscribers: cfe-commits, andrewrk, hans, srhines
Differential Revision: https://reviews.llvm.org/D67455