[runtimes] Shrink the set of runtimes included in the bootstrapping build by default

This patch is in preparation for removing libcxx, libcxxabi and libunwind
from LLVM_ENABLE_PROJECTS. When we make that switch, folks who were
previously using LLVM_ENABLE_PROJECTS=all in order to build those
runtimes will be able to add LLVM_ENABLE_RUNTIMES=all to start building
those projects using the bootstrapping build.

This is technically a breaking change for folks who had been using
LLVM_ENABLE_RUNTIMES=all, however I suspect the set of people currently
doing that is extremely small, so this is likely acceptable (more than
breaking folks who are using LLVM_ENABLE_PROJECTS=all).

Differential Revision: https://reviews.llvm.org/D132479

Author: ldionne

llvm/CMakeLists.txt

@@ -144,7 +144,10 @@ if ("flang" IN_LIST LLVM_ENABLE_PROJECTS)
 endif()
 
 # Select the runtimes to build
-set(LLVM_DEFAULT_RUNTIMES "compiler-rt;libc;libcxx;libcxxabi;libunwind;openmp")
+#
+# As we migrate runtimes to using the bootstrapping build, the set of default runtimes
+# should grow as we remove those runtimes from LLVM_ENABLE_PROJECTS above.
+set(LLVM_DEFAULT_RUNTIMES "libcxx;libcxxabi;libunwind")
 set(LLVM_SUPPORTED_RUNTIMES "libc;libunwind;libcxxabi;pstl;libcxx;compiler-rt;openmp;llvm-libgcc")
 set(LLVM_ENABLE_RUNTIMES "" CACHE STRING
   "Semicolon-separated list of runtimes to build, or \"all\" (${LLVM_DEFAULT_RUNTIMES}). Supported runtimes are ${LLVM_SUPPORTED_RUNTIMES}.")