[IR] Add interface to remove a CallBase string function attribute

[tejohnson]

Summary

Adds an interface to remove a string function attribute attached to a
CallBase, and a corresponding unittest.

This was extracted from D141077, and will be used by a follow on patch
that removes memprof attributes when needed.

[snehasish]

lgtm

llvm/unittests/IR/InstructionsTest.cpp

107

ASSERT_TRUE since it doesn't make sense to continue if we fail here.

[tejohnson]

Address comment