Clean up initialization of LangOptions object by splitting part of it into a parent class

'LangOptionsBase' that default initializes all flags to 0.  Now the ctor of LangOptions
only initializes the flags that might be non-zero (significantly simplifying things).

This also appeases Valgrind on Mac OS X, which no longer reports that one of the flags
may be uninitialized.

llvm-svn: 119045