[sanitizer] Implement include_if_exists with process name substitution.

[sanitizer] Implement include_if_exists with process name substitution.

include_if_exists=/path/to/sanitizer/options reads flags from the
file if it is present. "%b" in the include file path (for both
variants of the flag) is replaced with the basename of the main
executable.

llvm-svn: 242853