[runtimes] Add the libc project to the list of runtimes.

[sivachandra]

Summary

This is possible as the default libc build now works under runtimes build.

[phosek]

LGTM

llvm/runtimes/CMakeLists.txt

 # TODO: This file assumes the Clang toolchain so it'd be better if it lived in
 # Clang, except there already is clang/runtime directory which contains
 # similar although simpler functionality. We should figure out how to merge
 # the two files.
 
 # TODO: Selecting runtimes should be always performed inside the runtimes
 # build, see runtimes/CMakeLists.txt, except that we currently check whether
 # compiler-rt is being built to determine whether to first build builtins
 # or not so we need that information in this file as well.
-set(LLVM_ALL_RUNTIMES "compiler-rt;libcxx;libcxxabi;libunwind;openmp")
+set(LLVM_ALL_RUNTIMES "compiler-rt;libc;libcxx;libcxxabi;libunwind;openmp")
 set(LLVM_ENABLE_RUNTIMES "" CACHE STRING
   "Semicolon-separated list of runtimes to build (${LLVM_ALL_RUNTIMES}), or \"all\".")
 if(LLVM_ENABLE_RUNTIMES STREQUAL "all" )
   set(LLVM_ENABLE_RUNTIMES ${LLVM_ALL_RUNTIMES})
 endif()
 foreach(proj ${LLVM_ENABLE_RUNTIMES})
   set(proj_dir "${CMAKE_CURRENT_SOURCE_DIR}/../../${proj}")
   if(IS_DIRECTORY ${proj_dir} AND EXISTS ${proj_dir}/CMakeLists.txt)
if(compiler_rt_path)
   endif()
 endif()
 
 # We create a list the names of all the runtime projects in all uppercase and
 # with dashes turned to underscores. This gives us the CMake variable prefixes
 # for all variables that will apply to runtimes.
 foreach(entry ${runtimes})
   get_filename_component(projName ${entry} NAME)
+  if(projName STREQUAL "libc")
+    # For now, we will use the name "llvmlibc" for the libc project as it is
+    # not a full libc yet. Also, if we leave it as is, the "lib" prefix gets
+    # stripped below and the targets endup having the name "c", "check-c" etc.
+    set(projName "llvmlibc")
+  endif()
   string(REPLACE "-" "_" canon_name ${projName})
   string(TOUPPER ${canon_name} canon_name)
   list(APPEND prefixes ${canon_name})
   if (${canon_name} STREQUAL "OPENMP")
     list(APPEND prefixes "LIBOMP" "LIBOMPTARGET")
   endif()
   # Many compiler-rt options start with SANITIZER_ rather than COMPILER_RT_,
   # so when compiler-rt is enabled, consider both.

runtimes/CMakeLists.txt

 # This file handles building LLVM runtime sub-projects.
 cmake_minimum_required(VERSION 3.13.4)
 project(Runtimes C CXX ASM)
 
-set(LLVM_ALL_RUNTIMES "compiler-rt;libcxx;libcxxabi;libunwind;openmp")
+set(LLVM_ALL_RUNTIMES "compiler-rt;libc;libcxx;libcxxabi;libunwind;openmp")
 set(LLVM_ENABLE_RUNTIMES "" CACHE STRING
   "Semicolon-separated list of runtimes to build (${LLVM_ALL_RUNTIMES}), or \"all\".")
 if(LLVM_ENABLE_RUNTIMES STREQUAL "all" )
   set(LLVM_ENABLE_RUNTIMES ${LLVM_ALL_RUNTIMES})
 endif()
 
 foreach(proj ${LLVM_ENABLE_RUNTIMES})
   set(proj_dir "${CMAKE_CURRENT_SOURCE_DIR}/../${proj}")