[CMake] build correctly if build path contains whitespace

The add_llvm_symbol_exports function in AddLLVM.cmake creates command
line link flags with paths containing CMAKE_CURRENT_BINARY_DIR, but that
will break if CMAKE_CURRENT_BINARY_DIR contains whitespace. This patch
adds quotes to those paths.

Fixes PR39843.

Patch by John Garvin.

Differential Revision: https://reviews.llvm.org/D55081

llvm-svn: 347937