[flang] Document the Intrinsic Types

[sscalpone]

Summary

Describe the built-in integer, real, complex and logical
types implemented in flang, capturing the as-implemented
characteristics.

[PeteSteinfeld]

Thanks for doing this. Please be sure to correct the typo on line 79 before updating.

flang/docs/IntrinsicTypes.md

79

Should be "Modules compiled".

99

Do we want to discuss NAG's representation?

flang/docs/IntrinsicTypes.md

+<!--===- docs/IntrinsicTypes.md
+
+   Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+   See https://llvm.org/LICENSE.txt for license information.
+   SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+-->
+
+# Implementation of `Intrinsic` types in f18
+
+```eval_rst
+.. contents::
+   :local:
+```
+
+Intrinsic types are integer, real, complex, character, and logical.
+All intrinsic types have a kind type parameter called KIND,
+which determines the representation method for the specified type.
+The intrinsic type character also has a length type parameter called LEN,
+which determines the length of the character string.
+
+The implementation of `CHARACTER` type in f18 is described
+in [Character.md](Character.md).
+
+## Supported TYPES and KINDS
+
+Here are the type and kind combinations supported in f18:
+
+INTEGER(KIND=1) 8-bit two's-complement integer  
+INTEGER(KIND=2) 16-bit two's-complement integer  
+INTEGER(KIND=4) 32-bit two's-complement integer  
+INTEGER(KIND=8) 64-bit two's-complement integer  
+INTEGER(KIND=16) 128-bit two's-complement integer  
+
+REAL(KIND=2) 16-bit IEEE 754 binary16 (5e11m)  
+REAL(KIND=3) 16-bit upper half of 32-bit IEEE 754 binary32 (8e8m)  
+REAL(KIND=4) 32-bit IEEE 754 binary32 (8e24m)  
+REAL(KIND=8) 64-bit IEEE 754 binary64 (11e53m)  
+REAL(KIND=10) 80-bit extended precision with explicit normalization bit (15e64m)  
+REAL(KIND=16) 128-bit IEEE 754 binary128 (15e113m)  
+
+COMPLEX(KIND=2) Two 16-bit IEEE 754 binary16  
+COMPLEX(KIND=3) Two 16-bit upper half of 32-bit IEEE 754 binary32  
+COMPLEX(KIND=4) Two 32-bit IEEE 754 binary32  
+COMPLEX(KIND=8) Two 64-bit IEEE 754 binary64  
+COMPLEX(KIND=10) Two 80-bit extended precisions values  
+COMPLEX(KIND=16) Two 128-bit IEEE 754 binary128  
+
+No
+[double-double
+](https://en.wikipedia.org/wiki/Quadruple-precision_floating-point_format)
+quad precision type is supported.
+
+LOGICAL(KIND=1) 8-bit integer  
+LOGICAL(KIND=2) 16-bit integer  
+LOGICAL(KIND=4) 32-bit integer  
+LOGICAL(KIND=8) 64-bit integer  
+
+No 128-bit logical support.
+
+### Defaults kinds
+
+INTEGER 4  
+REAL 4  
+COMPLEX 4  
+DOUBLE PRECISION 8  
+LOGICAL 4  
+
+#### Modifying the default kind with default-real-8.  
+REAL 8  
+DOUBLE PRECISION  8  
+COMPLEX 8  
+
+#### Modifying the default kind with default-integer-8:  
+INTEGER 8
+
+There is no option to modify the default logical kind.
+
+Modules compile with different default-real and default-integer kinds
+may be freely mixed.
+Module files encode the kind value for every entity.
+
+## Representation of LOGICAL variables
+
+The default logical is `LOGICAL(KIND=4)`.
+
+Logical literal constants with kind 1, 2, 4, and 8
+share the following characteristics:   
+.TRUE. is represented as 1_kind  
+.FALSE. is represented as 0_kind  
+
+Tests for true is *integer value is not zero*.
+
+The implementation matches gfortran.
+
+Programs should not use integer values in LOGICAL contexts or
+use LOGICAL values to interface with other languages.
+
+### Representations of LOGICAL variables in other compilers
+
+##### Intel ifort / NVIDA nvfortran / PGI pgf90
+.TRUE. is represented as -1_kind  
+.FALSE. is represented as 0_kind  
+Any other values result in undefined behavior.  
+
+Values with a low-bit set are treated as .TRUE..  
+Values with a low-bit clear are treated as .FALSE..  
+
+##### IBM XLF
+.TRUE. is represented as 1_kind  
+.FALSE. is represented as 0_kind  
+
+Values with a low-bit set are treated as .TRUE..  
+Values with a low-bit clear are treated as .FALSE..