Fix an issue of getExtBuiltins()

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().

Test/spv.intrinsicsDebugBreak.frag

@@ -0,0 +1,9 @@
+#version 460
+#extension GL_EXT_spirv_intrinsics : enable
+
+spirv_instruction (extensions = ["SPV_KHR_non_semantic_info"], set = "NonSemantic.DebugBreak", id = 1)
+void debugBreak();
+
+void main() {
+  debugBreak();
+}