Move the name pointer out of Value into a map that lives on LLVMContext. Production builds of clang do not set names on Value's, so this is wasted space on almost all subclasses of Value. This reduces the size of all Value subclasses by 8 bytes on 64 bit hosts.
The one tricky part of this change is averting compile time regression by keeping Value::hasName() fast. This required stealing bits out of NumOperands.
With this change, peak memory usage on opt -O2 verify-uselistorder-nodbg.lto.bc is decreased by approximately 2.3% (~3MB absolute on my machine).