[CLANG][PATCH][FPEnv] Add support for option -ffp-eval-method and extend #pragma...

[CLANG][PATCH][FPEnv] Add support for option -ffp-eval-method and extend #pragma float_control similarly

The Intel compiler ICC supports the option "-fp-model=(source|double|extended)"
which causes the compiler to use a wider type for intermediate floating point
calculations. Also supported is a way to embed this effect in the source
program with #pragma float_control(source|double|extended).
This patch extends pragma float_control syntax, and also adds support
for a new floating point option "-ffp-eval-method=(source|double|extended)".
source: intermediate results use source precision
double: intermediate results use double precision
extended: intermediate results use extended precision

Reviewed By: Aaron Ballman

Differential Revision: https://reviews.llvm.org/D93769