IR: Extract a function 'printLLVMNameWithoutPrefix' from 'PrintLLVMName'. NFC.

This commit extracts the code that prints out a name of an LLVM value without a
prefix from a function 'PrintLLVMName' into a publicly accessible function named
'printLLVMNameWithoutPrefix'.

This change would be useful for MIR serialization, as it would allow the MIR
printer to reuse this function to print out the names of the external symbol
machine operands.

Reviewers: Duncan P. N. Exon Smith
llvm-svn: 242803