[cmake] Honor CMAKE_VERBOSE_MAKEFILE when building external projects (#75749)

When the top-level CMake invocation has `CMAKE_VERBOSE_MAKEFILE=ON`,
indicating the user wants to have verbose builds (i.e. all executed
commands explicitly echoed), some of the subprojects and runtimes (such
as compiler-rt, libcxx, etc) do not build in verbose mode. For example,
with Ninja:

```
[ 99% 6252/6308] cd /build/runtimes/builtins-bins && /usr/local/bin/cmake --build .
[  0% 6/308] Building C object CMakeFiles/clang_rt.builtins-i386.dir/absvti2.c.o
[  0% 7/308] Building C object CMakeFiles/clang_rt.builtins-i386.dir/absvdi2.c.o
[  0% 8/308] Building C object CMakeFiles/clang_rt.builtins-i386.dir/absvsi2.c.o
...
```

This is because `llvm_ExternalProject_Add()` and `add_custom_libcxx()`
use CMake's `ExternalProject_Add()` function to configure such
subproject builds, and do not pass through the `CMAKE_VERBOSE_MAKEFILE`
setting.

Similar to what is done in `clang/CMakeLists.txt`, add
`-DCMAKE_VERBOSE_MAKEFILE=ON` to the `ExternalProject_Add()` invocations
in `llvm_ExternalProject_Add()` and `add_custom_libcxx()`, whenever the
top-level CMake invocation had `CMAKE_VERBOSE_MAKEFILE` turned on.

Author: DimitryAndric

compiler-rt/cmake/Modules/AddCompilerRT.cmake

@@ -670,13 +670,18 @@ macro(add_custom_libcxx name prefix)
   get_property(CXX_FLAGS CACHE CMAKE_CXX_FLAGS PROPERTY VALUE)
   set(LIBCXX_CXX_FLAGS "${LIBCXX_CXX_FLAGS} ${CXX_FLAGS}")
 
+  if(CMAKE_VERBOSE_MAKEFILE)
+    set(verbose -DCMAKE_VERBOSE_MAKEFILE=ON)
+  endif()
+
   ExternalProject_Add(${name}
     DEPENDS ${name}-clobber ${LIBCXX_DEPS}
     PREFIX ${CMAKE_CURRENT_BINARY_DIR}/${name}
     SOURCE_DIR ${LLVM_MAIN_SRC_DIR}/../runtimes
     BINARY_DIR ${prefix}
     CMAKE_ARGS ${CMAKE_PASSTHROUGH_VARIABLES}
                ${compiler_args}
+               ${verbose}
                -DCMAKE_C_FLAGS=${LIBCXX_C_FLAGS}
                -DCMAKE_CXX_FLAGS=${LIBCXX_CXX_FLAGS}
                -DCMAKE_BUILD_TYPE=Release

llvm/cmake/modules/LLVMExternalProjectUtils.cmake

@@ -319,6 +319,10 @@ function(llvm_ExternalProject_Add name source_dir)
     list(APPEND compiler_args -DCMAKE_ASM_COMPILER_TARGET=${ARG_TARGET_TRIPLE})
   endif()
 
+  if(CMAKE_VERBOSE_MAKEFILE)
+    set(verbose -DCMAKE_VERBOSE_MAKEFILE=ON)
+  endif()
+
   ExternalProject_Add(${name}
     DEPENDS ${ARG_DEPENDS} llvm-config
     ${name}-clobber
@@ -330,6 +334,7 @@ function(llvm_ExternalProject_Add name source_dir)
     CMAKE_ARGS ${${nameCanon}_CMAKE_ARGS}
                --no-warn-unused-cli
                ${compiler_args}
+               ${verbose}
                -DCMAKE_INSTALL_PREFIX=${CMAKE_INSTALL_PREFIX}
                ${sysroot_arg}
                -DLLVM_BINARY_DIR=${PROJECT_BINARY_DIR}