[CMake] Port add_sanitizer_rt_symbols to CMake 3.0

Patch by Brad King.

Our add_sanitizer_rt_symbols macro reads the LOCATION property of a
library to compute the location of the "lib<name>.a.syms" file to
generate next to the corresponding "lib<name>.a" library file.  CMake
3.0 introduces policy CMP0026 to disallow reading of the LOCATION target
property from non-imported targets in favor of the more powerful
$<TARGET_FILE> generator expression.

Teach add_sanitizer_rt_symbols to use the $<TARGET_FILE> generator
expression to compute the location of the symbols file to generate
with a custom command.  CMake 3.0 also adds support for generator
expressions to install(FILES) so use it when available to simplify
installation of the symbols file of the proper configuration.

llvm-svn: 202797