🏗🐛 Do not generate code during gulp check-types

[rsimha]

With this PR, we no longer generate code during gulp check-types. Specifically, all these files are no longer written to the dist/ folder:

rsimha@rsimha-macbookpro2 (master):~/src/amphtml$ ls -lar dist/
total 15384
-rw-r--r--   1 rsimha  primarygroup   327472 Jul 17 10:25 integration-check-types.js.map
-rw-r--r--   1 rsimha  primarygroup   172418 Jul 17 10:25 integration-check-types.js
-rw-r--r--   1 rsimha  primarygroup    37574 Jul 17 10:25 iframe-transport-client-check-types.js.map
-rw-r--r--   1 rsimha  primarygroup    16709 Jul 17 10:25 iframe-transport-client-check-types.js
-rw-r--r--   1 rsimha  primarygroup  4144583 Jul 17 10:25 check-types.js.map
-rw-r--r--   1 rsimha  primarygroup  3070012 Jul 17 10:25 check-types.js
-rw-r--r--   1 rsimha  primarygroup    63058 Jul 17 10:25 ampcontext-check-types.js.map
-rw-r--r--   1 rsimha  primarygroup    27957 Jul 17 10:25 ampcontext-check-types.js
drwxr-xr-x  50 rsimha  primarygroup     1600 Jul 17 10:25 ../
drwxr-xr-x  10 rsimha  primarygroup      320 Jul 17 10:25 ./

Notes:

• I've confirmed that type errors are still correctly detected with this change
• The original issue suggests that we no longer run SIMPLE_OPTIMIZATIONS during type checking, but it turns out that we can't use WHITESPACE_ONLY optimization according to this comment:
  

  amphtml/build-system/tasks/compile.js

  Lines 344 to 347 in 703aad7

  	if (argv.typecheck_only || checkTypes) {
  	// Don't modify compilation_level to a lower level since
  	// it won't do strict type checking if its whitespace only.
  	compilerOptions.compilerFlags.define.push('TYPECHECK_ONLY=true');

Fixes #5024

[rsimha]

/to @erwinmombay @cramforce

[codecov-io]

Codecov Report

Merging #16799 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #16799      +/-   ##
==========================================
+ Coverage   78.01%   78.06%   +0.04%     
==========================================
  Files         558      558              
  Lines       40440    40440              
==========================================
+ Hits        31551    31571      +20     
+ Misses       8889     8869      -20

Flag	Coverage Δ
#integration_tests	35.14% <ø> (+0.28%)	⬆️
#unit_tests	77.11% <ø> (+0.05%)	⬆️

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