[clang][NFC] Extract the EmitAssemblyHelper::TargetTriple member

Few times in different methods of the EmitAssemblyHelper class the following
code snippet is used to get the TargetTriple and then use it's single method
to check some conditions:

TargetTriple(TheModule->getTargetTriple())

The parsing of a target triple string is not a trivial operation and it takes
time to repeat the parsing many times in different methods of the class and
even numerous times in one method just to call a getter
(llvm::Triple(TheModule->getTargetTriple()).getVendor()), for example.
The patch extracts the TargetTriple member of the EmitAssemblyHelper class to
parse the triple only once in the class' constructor.

Reviewed By: tejohnson

Differential Revision: https://reviews.llvm.org/D122587