This patch is part of a patch series to add support for FileCheck
numeric expressions. This specific patch gives earlier and better
diagnostics for the -D option.
Prior to this change, parsing of -D option was very loose: it assumed
that the part on the left of the equal sign was a valid variable name.
This commit adds logic to ensure that this is the case and gives
diagnostic when it is not, making it clear that the issue came from a
command-line option error. This is achieved by sharing the variable
parsing code into a new function ParseVariable.