[llvm-tapi] Don't try to override SequenceTraits for std::string

For some reason this doesn't seem to work with LLVM_LINK_LLVM_DYLIB
build.

See https://logs.chromium.org/logs/chromium/bb/client.wasm.llvm/linux/37764/+/recipes/steps/LLVM_regression_tests/0/stdout

What is more it seems that overriding these traits for core types
(including std::string) is not supported/recommend by YAMLTraits.h.
See line 1918 which has the assertion:
 "only use LLVM_YAML_IS_SEQUENCE_VECTOR for types you control"

Differential Revision: https://reviews.llvm.org/D55381

llvm-svn: 348630