Switch the X86TargetInfo object from a string representation of the

selected CPU model to the enumeration. This parses the string
representation once using a StringSwitch on SetCPU. It returns an error
for strings which are not recognized (yay!). Finally it replaces
ridiculous if-chains with switches that cover all enumerators.

The last change required adding several missing entries to the features
function. These were obvious on inspection. Yay for a pattern that gives
warnings when we miss one.

No new test cases yet, as I want to get the 64-bit errors working first.
I'll then start fleshing out the testing more. Currently I'm primarily
testing on Linux, but I'm hoping check whether there are interesting
differences on darwin before long...

llvm-svn: 140685