MemorySanitizer does not require PIE.

Since r249754 MemorySanitizer should work equally well for PIE and
non-PIE executables on Linux/x86_64.

Beware, with this change -fsanitize=memory no longer adds implicit
-fPIE -pie compiler/linker flags on Linux/x86_64.

This is a re-land of r250941, limited to Linux/x86_64 + a very minor
refactoring in SanitizerArgs.

llvm-svn: 250949