Replace redundant code in FormatManager and FormatCache with templates (NFC)

This is a preparatory patch for an upcoming bugfix.

FormatManager and friends have four identical implementations of many
accessor functions to deal with the four types of shared pointers in
the FormatCache. This patch replaces these implementations with
templates. While this patch drastically reduces the amount of source
code and its maintainablity, it doesn't actually improve code
size. I'd argue, this is still an improvement.

rdar://problem/57756763

Differential Revision: https://reviews.llvm.org/D71231