[support] Revise ScopedPrinter formatting tests for floats

Previously there were several attempts to make the format checks for NaN
and Inf work across platforms, like AIX and Solaris, that print these
values slightly differently. This resulted in a number of forward fixes,
until we finally disabled the tests for NaN and Inf. This change should
make the test robust across different platforms, and reduce the overall
amount of code by delegating to helper functions that use the same
format strings as the implementations used by PrintNumber().

This additionally reverts commit 5a9bad17
and fa56e362.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D146851