[AST, analyzer] Transform rvalue cast outputs to lvalues (fheinous-gnu-extensions)

Despite the fact that cast expressions return rvalues, GCC still
handles such outputs as lvalues when compiling inline assembler.
In this commit, we are treating it by removing LValueToRValue
casts inside GCCAsmStmt outputs.

Differential Revision: https://reviews.llvm.org/D45416

llvm-svn: 344864