MIR Parser: Rename the method 'diagFromLLVMAssemblyDiag'. NFC.

This commit renames the method 'diagFromLLVMAssemblyDiag' to
'diagFromBlockStringDiag'. This method will be used when converting diagnostics
from other YAML block strings, and not just the LLVM module block string, so
the new name should reflect that.

llvm-svn: 244943