This function is used to import SPIR-V extended instruction set. It
mistakenly treated the name of SPIR-V extended instruction set as the
name of SPIR-V extension. For example, when we have such code
getExtBuiltins("NonSemantic.DebugBreak")
'NonSemantic.DebugBreak' is added to SPIR-V extension. Rather, the
SPIR-V extension name should be 'SPV_KHR_non_semantics_info'. Therefore,
we must avoid this since the name of SPIR-V extended instruction set is
not necessarily equal to that of relevant SPIR-V extension. Adding a
SPIR-V extension must be done by calling addExtension() explicitly
outside this function.
This change also fixes disassembly issues of debugBreak().
