This patch enables Function Specialization by default.
Compilation Time Overhead:
--------------------------
Measured the Instruction Count increase (Geomean) for CTMark from
the llvm-testsuite as in https://llvm-compile-time-tracker.com.
 * {-O3, Non-LTO}: +0.136% Instruction Count
 * {-O3, LTO}: +0.346% Instruction Count
Performance Uplift:
-------------------
Measured +9.121% score increase for 505.mcf_r from SPEC Int 2017
(Tested on Neoverse N1 with -O3 + LTO)
Correctness Testing:
--------------------
 * Passes bootstrap Clang with ASAN + LTO + FuncSpec aggressive options:
   { MaxClonesThreshold=10,
     SmallFunctionThreshold=10,
     AvgLoopIterationCount=30,
     SpecializeOnAddresses=true,
     EnableSpecializationForLiteralConstant=true,
     FuncSpecializationMaxIters=10 }
 * Builds Chromium and passes its unittests with the above options + ThinLTO.
For more info please refer to
https://discourse.llvm.org/t/rfc-should-we-enable-function-specialization/61518