Support merge_functions option in NewPM since LLVM >= 12
Support merge_functions option in NewPM since LLVM >= 12

now we can pass this flag since https://reviews.llvm.org/D93002 has been
merged.

aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust