Fix #1924: Promote SPV_EXT_physical_storage_buffer to KHR when required.

If the semantics that require the KHR form over the EXT form are seen
(OpBitcast between a vector and a pointer) promote the requested extension
from the EXT to the KHR.

SPIRV/SpvBuilder.h

@@ -106,11 +106,20 @@ public:
     void addModuleProcessed(const std::string& p) { moduleProcesses.push_back(p.c_str()); }
     void setEmitOpLines() { emitOpLines = true; }
     void addExtension(const char* ext) { extensions.insert(ext); }
+    void removeExtension(const char* ext)
+    {
+        extensions.erase(ext);
+    }
     void addIncorporatedExtension(const char* ext, SpvVersion incorporatedVersion)
     {
         if (getSpvVersion() < static_cast<unsigned>(incorporatedVersion))
             addExtension(ext);
     }
+    void promoteIncorporatedExtension(const char* baseExt, const char* promoExt, SpvVersion incorporatedVersion)
+    {
+        removeExtension(baseExt);
+        addIncorporatedExtension(promoExt, incorporatedVersion);
+    }
     void addInclude(const std::string& name, const std::string& text)
     {
         spv::Id incId = getStringId(name);