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

Change the ELF YAML implementation of TextAPI so NeededLibs uses flow
sequence vector correctly instead of overriding the YAML implementation
for std::vector<std::string>>.

This should fix the test failure with the LLVM_LINK_LLVM_DYLIB build mentioned in D55381.

Still passes existing tests that cover this.

Differential Revision: https://reviews.llvm.org/D55390

llvm-svn: 348551