[CMAKE] Provide default location for llvm-lit for out-of-tree users.
Several external build users contain some heuristics for finding llvm-lit.
There are several cases we need to worry about:
- External builds against a build tree (with LLVM_BUILD_UTILS)
- External builds against an install tree (with LLMV_BUILD_UTIL
and LLVM_INSTALL_UTILS)
- External builds against some location which doesn't have an
llvm-lit, but llvm-lit is available through some other means, such
as an available source tree, or a packager provided llvm-lit.
For the third case, LLVM_EXTERNAL_LIT suffices, but in other cases
there's no standard way to find llvm-lit. It seems like each user
cooks their own heuristics:
- clang tries to look in the LLVM source tree, and failing that falls
back to looking for a packaged llvm-lit.
- libcxx tries to look in the LLVM source tree, which might come from
llvm-config or be explicitly specified.
This patch is a first stop to solving this by providing a default location
for llvm-lit using LLVM_DEFAULT_EXTERNAL_LIT. The expectation is that
future patches can clean up users like clang and libcxx to rely
on this mechanism for out-of-tree builds.
Differential Revision: https://reviews.llvm.org/D77110