Currently, when the -relink-builtin-bitcodes-postop option is used we
link builtin bitcodes twice: once before optimization, and again after
optimization.
With this change, we omit the pre-opt linking when the option is set,
and we rename the option to the following:
-Xclang -mlink-builtin-bitcodes-postopt
(-Xclang -mno-link-builtin-bitcodes-postopt)
The goal of this change is to reduce compile time. We do lose the
theoretical benefits of pre-opt linking, but in practice these are small
than the overhead of linking twice. However we may be able to address
this in a future patch by adjusting the position of the builtin-bitcode
linking pass.
Compilations not setting the option are unaffected
