[CodeGen] Tweak isTriviallyRecursive further

[CodeGen] Tweak isTriviallyRecursive further

isTriviallyRecursive is a hack used to bridge a gap between the
expectations that source code assumes and the semantics that LLVM IR can
provide.  Specifically, asm labels on functions are treated as an
explicit name for a GlobalObject in Clang but treated like an
output-processing step in GCC.  Tweak this hack a little further to emit
calls to library functions instead of emitting an incorrect definition.
The definition in question would have available_externally linkage (this
is OK) but result in a call to itself which will either result in an
infinite loop or stack overflow.

This fixes PR23964.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@241043 91177308-0d34-0410-b5e6-96231b3b80d8