[release] Build the LLVM releases with Position Independent Code enabled

[ldionne]

Summary

According to http://llvm.org/PR43604, it looks like we want to provide
the guarantee that users can build their applications with -fPIC or -fPIE
when linking against libc++.a. This is only possible if we build libc++.a
with -fPIC itself, which can be achieved through CMake's
CMAKE_POSITION_INDEPENDENT_CODE knob.

[ldionne]

I created another take on this here: https://reviews.llvm.org/D104328.

Personally, I find the solution in this patch here more elegant, since it avoids hardcoding that choice in a layer way below in libc++.

[ldionne]

Abandoning since I went with D104328 instead.

llvm/utils/release/test-release.sh

function configure_llvmCore() {
 
     cd $ObjDir
     echo "# Configuring llvm $Release-$RC $Flavor"
 
     echo "#" env CC="$c_compiler" CXX="$cxx_compiler" \
         cmake -G "$generator" \
         -DCMAKE_BUILD_TYPE=$BuildType -DLLVM_ENABLE_ASSERTIONS=$Assertions \
         -DLLVM_ENABLE_PROJECTS="$project_list" \
+        -DCMAKE_POSITION_INDEPENDENT_CODE=ON \
         $ExtraConfigureFlags $BuildDir/llvm-project/llvm \
         2>&1 | tee $LogDir/llvm.configure-Phase$Phase-$Flavor.log
     env CC="$c_compiler" CXX="$cxx_compiler" \
         cmake -G "$generator" \
         -DCMAKE_BUILD_TYPE=$BuildType -DLLVM_ENABLE_ASSERTIONS=$Assertions \
         -DLLVM_ENABLE_PROJECTS="$project_list" \
+        -DCMAKE_POSITION_INDEPENDENT_CODE=ON \
         $ExtraConfigureFlags $BuildDir/llvm-project/llvm \
         2>&1 | tee $LogDir/llvm.configure-Phase$Phase-$Flavor.log
 
     cd $BuildDir
 }
 
 function build_llvmCore() {
     Phase="$1"