gccrs: Added AST Node AST::InlineAsm

Addresses #1567
Created a AST node InlineAsm similar to the one found in rustc.
As there is no Symbol struct/class in gccrs I have made every instance
of Symbol a string.

gcc/rust/ChangeLog:

	* ast/rust-ast-full-decls.h (class InlineAsm):Added class declaration.
	* ast/rust-expr.h (class InlineAsm):Added class definition.

Signed-off-by: M V V S Manoj Kumar <mvvsmanojkumar@gmail.com>