[CMake] install-distribution for LLDB on Darwin
There's a number of requirements for installing LLDB on macOS that are untypical for LLVM projects: use special install-prefix for LLDB.framework, ship headers and tools as framework resources, patch RPATHs, externalize debug-info to dSYM's and strip binaries with -ST. For some of it we could use llvm_externalize_debuginfo() in the past and just add special cases. However, this complicates the code for all projects and comes with the major drawback, that it adds all these actions at build-time, i.e. dSYM creation and stripping take a lot of time and don't make sense at build-time.
LLVM's distribution mechanism (https://llvm.org/docs/BuildingADistribution.html) appears to be the natural candidate to install LLDB. Based on D64399 (enable in standalone builds), this patch integrates framework installation with the distribution mechanism and adds custom stripping flags and dSYM creation at install-time. Unlike the abandoned D61952, it leaves build-tree binaries untouched, so there's no side-effects on testing. Potential install-order issues must be handled externally.
Please let me know what you think, while I run a few more tests and add remarks+documentation.
Reviewers: xiaobai, compnerd, JDevlieghere, davide, labath, mgorny
Reviewed By: xiaobai, JDevlieghere
Subscribers: lldb-commits, #lldb
Differential Revision: https://reviews.llvm.org/D64408