[libc++] Take 2: Do not cleverly link against libc++abi just because it happens to be there

Otherwise, when libcxxabi is not an enabled project in the monorepo, we
get a link error because we try to link against non-existent cxxabi_shared.

More generally, we shouldn't change the behavior of the build based on
implicit things like whether a file happens to be at a specific path or

This is a re-application of r365222 that had been reverted in r365233
because it broke the build bots. However, the build bots now specify
explicitly what ABI library they want to use (libc++abi), so this
commit should now be OK to merge.

Differential Revision: https://reviews.llvm.org/D63883

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@365326 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed