[CMake] Add option to link LLVM/subproject executables against LLVM libc

This patch adds in CMake option LLVM_ENABLE_LLVM_LIBC which when set to
true automatically builds LLVM libc in overlay mode and links all
generated executables against the libc overlay. This is intended to
somewhat mirror the LLVM_ENABLE_LIBCXX flag.

Differential Revision: https://reviews.llvm.org/D151013

Author: boomanaiden154

llvm/CMakeLists.txt

@@ -558,6 +558,7 @@ else()
 endif()
 option(LLVM_ENABLE_LOCAL_SUBMODULE_VISIBILITY "Compile with -fmodules-local-submodule-visibility." ON)
 option(LLVM_ENABLE_LIBCXX "Use libc++ if available." OFF)
+option(LLVM_ENABLE_LLVM_LIBC "Set to on to link all LLVM executables against LLVM libc, assuming it is accessible by the host compiler." OFF)
 option(LLVM_STATIC_LINK_CXX_STDLIB "Statically link the standard library." OFF)
 option(LLVM_ENABLE_LLD "Use lld as C and C++ linker." OFF)
 option(LLVM_ENABLE_PEDANTIC "Compile with pedantic enabled." ON)

llvm/cmake/modules/AddLLVM.cmake

@@ -1029,6 +1029,10 @@ macro(add_llvm_executable name)
     target_link_libraries(${name} PRIVATE ${LLVM_PTHREAD_LIB})
   endif()
 
+  if(HAVE_LLVM_LIBC)
+    target_link_libraries(${name} PRIVATE llvmlibc)
+  endif()
+
   llvm_codesign(${name} ENTITLEMENTS ${ARG_ENTITLEMENTS} BUNDLE_PATH ${ARG_BUNDLE_PATH})
 endmacro(add_llvm_executable name)
 

llvm/cmake/modules/HandleLLVMOptions.cmake

@@ -1300,3 +1300,10 @@ endif()
 
 set(LLVM_THIRD_PARTY_DIR  ${CMAKE_CURRENT_SOURCE_DIR}/../third-party CACHE STRING
     "Directory containing third party software used by LLVM (e.g. googletest)")
+
+if(LLVM_ENABLE_LLVM_LIBC)
+  check_library_exists(llvmlibc printf "" HAVE_LLVM_LIBC)
+  if(NOT HAVE_LLVM_LIBC)
+    message(WARNING "Unable to link against LLVM libc. LLVM will be built without linking against the LLVM libc overlay.")
+  endif()
+endif()

llvm/docs/CMake.rst

@@ -514,6 +514,11 @@ enabled sub-projects. Nearly all of these variable names begin with
   passed to invocations of both so that the project is built using libc++
   instead of stdlibc++. Defaults to OFF.
 
+**LLVM_ENABLE_LLVM_LIBC**: BOOL
+  If the LLVM libc overlay is installed in a location where the host linker
+  can access it, all built executables will be linked against the LLVM libc
+  overlay before linking against the system libc. Defaults to OFF.
+
 **LLVM_ENABLE_LIBPFM**:BOOL
   Enable building with libpfm to support hardware counter measurements in LLVM
   tools.