Fix filesystem build on platforms with weird time_t types.

32-bit powerpc provides a 64 bit time_t type and older ppc64 systems
provide time_t as a floating point type. This caused problems when building
operations.cpp since operations.cpp contained compile time tests for conversions
between time_t and filesystem time type.

When these tests failed they caused the libc++ build to fail as well. This is unfortunate.

This patch moves the tests out of the source file and into the test suite. It also
expands the tests to allow testing of the weird time_t configurations on all platforms.

llvm-svn: 307461