[clang][Sema] Shard out SemaTests into their own binary (llvm#155548)

Co-authored-by: Reid Kleckner <[email protected]>

Author: HighCommander4

clang/unittests/Sema/CMakeLists.txt

@@ -1,4 +1,8 @@
-add_clang_unittest(SemaTests
+# SemaTests are distinct to enable fast incremental builds.
+# Merging it into AllClangTests would result in one less
+# large statically linked binary, but separating it out is
+# the right tradeoff today.
+add_distinct_clang_unittest(SemaTests
   ExternalSemaSourceTest.cpp
   CodeCompleteTest.cpp
   HeuristicResolverTest.cpp