[CMake][libc] Fix non-runtime build when other runtimes are enabled

[aidengrossman]

Summary

Before this patch, when other runtimes were enabled by setting
LLVM_ENABLE_RUNTIMES and llvm libc was built as a project by setting
LLVM_ENABLE_PROJECTS, the llvm libc CMake system would delay
configuration until the runtime build which never started since libc
wasn't declared as one of the runtime builds. This patch fixes this
behavior by explicitly checking that libc is within LLVM_ENABLE_RUNTIMES
rather than just the variable being set at all.

libc/CMakeLists.txt

if(NOT LIBC_HDRGEN_EXE)
     # We need to set up hdrgen first since other targets depend on it.
     add_subdirectory(utils/LibcTableGenUtil)
     add_subdirectory(utils/HdrGen)
   else()
     message(STATUS "Will use ${LIBC_HDRGEN_EXE} for libc header generation.")
   endif()
 endif()
 
-if(LLVM_ENABLE_RUNTIMES AND NOT LLVM_RUNTIMES_BUILD)
+if("libc" IN_LIST LLVM_ENABLE_RUNTIMES AND NOT LLVM_RUNTIMES_BUILD)
   # When libc is build as part of the runtimes/bootstrap build's CMake run, we
   # only need to build the host tools to build the libc. So, we just do enough
   # to build libc-hdrgen and return.
   return()
 endif()
 
 option(LIBC_CMAKE_VERBOSE_LOGGING
        "Log details warnings and notifications during CMake configuration." OFF)