Move the name pointer out of Value into a map that lives on the

LLVMContext.  Production builds of clang do not set names on most
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 verify-uselistorder-nodbg.lto.bc
is decreased by approximately 2.3% (~3MB absolute on my machine).

llvm-svn: 238791