Implemented support for "pragma clang optimize on/off", based on attribute 'optnone'.

This patch implements support for selectively disabling optimizations on a
range of function definitions through a pragma. The implementation is that
all function definitions in the range are decorated with attribute
'optnone'.

    #pragma clang optimize off
    // All function definitions in here are decorated with 'optnone'.
    #pragma clang optimize on
    // Compilation resumes as normal.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209510 91177308-0d34-0410-b5e6-96231b3b80d8