[llvm-lib] Ignore /LTCG option

"The /LTCG option to LIB specifies that the inputs from cl.exe include object
files generated by using the /GL compiler option."

Based on Microsoft's description
above (https://docs.microsoft.com/en-us/cpp/build/reference/running-lib?view=msvc-170),
there doesn't seem to be anything llvm-lib needs to do to support the flag.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D126000