Add release note for DIBuilder API changes

MCJIT and ExecutionEngine continue to be supported, though ORC should be

preferred for new projects.

Changes to the C++ APIs

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Three of the IR library methods related to debugging information for

functions and methods have changed their prototypes:

  DIBuilder::createMethod

  DIBuilder::createFunction

  DIBuilder::createTempFunctionFwdDecl

In all cases, several individual parameters were removed, and replaced

by a single 'SPFlags' (subprogram flags) parameter. The individual

parameters are: 'isLocalToUnit'; 'isDefinition'; 'isOptimized'; and

for 'createMethod', 'Virtuality'.  The new 'SPFlags' parameter has a

default value equivalent to passing 'false' for the three 'bool'

parameters, and zero (non-virtual) to the 'Virtuality' parameter.  For

any old-style API call that passed 'true' or a non-zero virtuality to

these methods, you will need to substitute the correct 'SPFlags' value.

The helper method 'DISubprogram::toSPFlags()' might be useful in making

this conversion.

Changes to the AArch64 Target

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