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.
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