[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
were stale.

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 everytime the HEAD commit
changes (due to commit hash being embedded in the output of `--version`)
which means all of compiler-rt will be rebuilt everytime this happens.
While this is annoying it's better to do the slow but correct thing
rather than the fast but incorrect thing.

rdar://75150660

Differential Revision: https://reviews.llvm.org/D98291

clang/runtime/CMakeLists.txt

@@ -95,6 +95,8 @@ if(LLVM_BUILD_EXTERNAL_COMPILER_RT AND EXISTS ${COMPILER_RT_SRC_ROOT}/)
     USES_TERMINAL_CONFIGURE 1
     USES_TERMINAL_BUILD 1
     USES_TERMINAL_INSTALL 1
+    # Always run the build command so that incremental builds are correct.
+    BUILD_ALWAYS 1
     )
 
   get_ext_project_build_command(run_clean_compiler_rt clean)