Attempt to fix build on Windows with LLVM_ENABLE_PIC=OFF

libclang can be built in shared or static mode. On Windows, with
LLVM_ENABLE_PIC=OFF, it was built in neither mode, leading to clients of
libclang (c-index-test, c-arcmt-test) failing to link with it set.

Since PIC isn't really a thing on Windows, build libclang in shared mode when
LLVM_ENABLE_PIC=OFF there. This is also somewhat symmetric with the existing
ENABLE_STATIC a few lines down.

Differential Revision: https://reviews.llvm.org/D57258

llvm-svn: 352253