[sanitizers] Sanitizer tests CMake clean up

This patch addresses two issues:

Most of the time, hacks with `if/else` in order to get support for
multi-configuration builds are superfluous.
The variable `CMAKE_CFG_INTDIR` was created precisely for this purpose: it
expands to `.`  on all single-configuration builds, and to a configuration
name otherwise.
The `if/else` hacks for the library name generation should also not be
done, as CMake has `TARGET_FILE` generator expression precisely for this
purpose, as it expands to the exact filename of the resulting target.

Differential Revision: https://reviews.llvm.org/D35952

llvm-svn: 309306