[libc] Provide an implementation of the 'stdint.h' header

Summary:
I've noticed one problem is that the user includes `stdint.h` the
compiler will do `#include_next <stdint.h>` potentially into a
conflicting implementation on systems with multiple headers installed.
The `clang` header is standards compliant and works with `clang` and
`gcc` which are both of our targets, so I simply copied it here. This
has the effect of including `stdint.h` on clang / LLVM libc behaving the
same as `-ffreestanding`.

Author: jhuber6

libc/include/CMakeLists.txt

@@ -84,6 +84,14 @@ add_gen_header(
     .llvm-libc-macros.float_macros
 )
 
+add_gen_header(
+  stdint
+  DEF_FILE stdint.h.def
+  GEN_HDR stdint.h
+  DEPENDS
+    .llvm-libc-macros.stdint_macros
+)
+
 add_gen_header(
   limits
   DEF_FILE limits.h.def

libc/include/llvm-libc-macros/CMakeLists.txt

@@ -67,6 +67,12 @@ add_macro_header(
     file-seek-macros.h
 )
 
+add_macro_header(
+  stdint_macros
+  HDR
+    stdint-macros.h
+)
+
 add_macro_header(
   float_macros
   HDR