[compiler-rt] Fix stale incremental builds when using LLVM_BUILD_EXTERNAL_COMPILER_RT=ON.
When building with LLVM_BUILD_EXTERNAL_COMPILER_RT=ON (e.g. Swift does
this) we do an "external" build of compiler-rt where we build
compiler-rt with the just built clang.
Unfortunately building in this mode had a bug where compiler-rt would
not get rebuilt if compiler-rt sources changed. This is problematic
for incremental builds because it meant that the compiler-rt binaries
The fix is to use the BUILD_ALWAYS ExternalProject_Add option which
means the build command for compiler-rt is always run.
In principle if all of the following are true:
- compiler-rt has already been built.
- there are no compiler-rt source changes.
- the compiler hasn't changed.
- ninja is being used as the generator for the compiler-rt build.
then the overhead for always running the build command for incremental
builds is negligible.
However, in practice clang gets rebuilt every time the HEAD commit
changes (due to commit hash being embedded in the output of --version)
which means all of compiler-rt will be rebuilt every time this happens.
While this is annoying it's better to do the slow but correct thing
rather than the fast but incorrect thing.