Update all LLVM documentation mentioning runtimes in LLVM_ENABLE_PROJECTS

We are moving away from building the runtimes with LLVM_ENABLE_PROJECTS,
however the documentation was largely outdated. This commit updates all
the documentation I could find to use LLVM_ENABLE_RUNTIMES instead of
LLVM_ENABLE_PROJECTS for building runtimes.

Note that in the near future, libcxx, libcxxabi and libunwind will stop
supporting being built with LLVM_ENABLE_PROJECTS altogether. I don't know
what the plans are for other runtimes like libc, openmp and compiler-rt,
so I didn't make any changes to the documentation that would imply
something for those projects.

Once this lands, I will also cherry-pick this on the release/14.x branch
to make sure that LLVM's documentation is up-to-date and reflects what
we intend to support in the future.

Differential Revision: https://reviews.llvm.org/D119351

bolt/docs/OptimizingClang.md

@@ -41,7 +41,7 @@ Collecting accurate profile requires running `perf` on a hardware that
 implements taken branch sampling (`-b/-j` flag). For that reason, it may not be possible to
 collect the accurate profile in a virtualized environment, e.g. in the cloud.
 We do support regular sampling profiles, but the performance
-improvements are expected to be more modest. 
+improvements are expected to be more modest.
 
 ```bash
 $ mkdir ${TOPLEV}/stage3
@@ -211,7 +211,8 @@ $ cd ${TOPLEV}/stage1
 $ cmake -G Ninja ${TOPLEV}/llvm-project/llvm -DLLVM_TARGETS_TO_BUILD=X86 \
       -DCMAKE_BUILD_TYPE=Release \
       -DCMAKE_C_COMPILER=gcc -DCMAKE_CXX_COMPILER=g++ -DCMAKE_ASM_COMPILER=gcc \
-      -DLLVM_ENABLE_PROJECTS="clang;lld;compiler-rt" \
+      -DLLVM_ENABLE_PROJECTS="clang;lld" \
+      -DLLVM_ENABLE_RUNTIMES="compiler-rt" \
       -DCOMPILER_RT_BUILD_SANITIZERS=OFF -DCOMPILER_RT_BUILD_XRAY=OFF \
       -DCOMPILER_RT_BUILD_LIBFUZZER=OFF \
       -DCMAKE_INSTALL_PREFIX=${TOPLEV}/stage1/install