[CIR][NFC] Consildate CIRGenExprCXX.cpp files (#157169)

Somewhere in the upstreaming process, we created a file
CIRGenCXXExpr.cpp that corresponded to the file CIRGenExprCXX.cpp in the
incubator. Later we created a CIRGenExprCXX.cpp file.

This change consolidates those files, keeping the name used in the
incubator.

Author: andykaylor