Fix tests to be more robust (to older versions of grep, lesser lit-like test runners, etc)

Seems I had a problem with my version of grep, when run by lit, not supporting
the \s escape. This seems to fix it for me & I'll be getting the buildbots to
run these tests too to keep an eye on them (actually loop-convert tests still
fail when run via a make build, so that'll be addressed in a future commit). I
could use [[:space:]] to generalize over other whitespace but that seemed
unnecessarily verbose when the flexibility wasn't actually required by the
current text of the tests.

Also I just simplified a lot of the loop-convert tests (removing the
unecessary temp file deletion at the start, removing the unnecessary && for
FileCheck, etc).

The remove-cstr-calls/basic.cpp changes were necessitated by an out of tree
lit-like test runner that's a bit less fantastic about escaping. They were
modeled on existing tooling test cases in Clang, with thanks to Manuel Klimek
for the pointers.

llvm-svn: 163009