Fix integer overflow in PCHReader when reading the length of an

identifier.  This caused a crash when reading PCH files that contained
long identifier names.

The issue is that 'StrLenPtr' was previously a 'const char *', meaning
the byte loaded from it would be interpretted as a signed integer.  If
the topmost bit was set, conversion to 'unsigned' would extend that
bit, causing an overflow.

The solution is to make 'StrLenPtr' an 'unsigned char *', always
treating the value as an unsigned integer.

This fixes: <rdar://problem/7328900>

llvm-svn: 84925