Results in a 7.5% bigger RelWithDebInfo ldc2.exe on Win64 with LLVM 3.9.1.
LLD is currently enforced when building with LDC_WITH_LLD=ON. And LLD
still doesn't support debuginfo (.pdb) generation for MSVC targets.
By directly integrating LLVM's `llvm-lib.exe` driver to generate static
libs for MSVC targets and a stripped-down version of the `llvm-ar` tool
for the other targets.
Introduce command-line option `-archiver=<file>` to allow the user to
specify an external archiver to be invoked.
