GROOVY-9381: Support async/await like ES7

[daniellansun]

https://issues.apache.org/jira/browse/GROOVY-9381

[codecov-commenter]

Codecov Report

:x: Patch coverage is 75.82418% with 44 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 67.0320%. Comparing base (4e0dc1a) to head (5e7e868). :warning: Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
...va/groovy/util/concurrent/async/SimplePromise.java	83.3333%	16 Missing :warning:
...java/groovy/util/concurrent/async/AsyncHelper.java	70.0000%	5 Missing and 1 partial :warning:
...a/groovy/util/concurrent/async/AwaitException.java	25.0000%	6 Missing :warning:
...ain/java/groovy/util/concurrent/async/Promise.java	0.0000%	5 Missing :warning:
...a/groovy/util/concurrent/async/StageAwaitable.java	0.0000%	5 Missing :warning:
...ehaus/groovy/transform/AsyncASTTransformation.java	80.9524%	2 Missing and 2 partials :warning:
...va/org/apache/groovy/parser/antlr4/AstBuilder.java	88.8889%	1 Missing and 1 partial :warning:

Additional details and impacted files

@@                Coverage Diff                 @@
##               master      #2324        +/-   ##
==================================================
+ Coverage     67.0088%   67.0320%   +0.0231%     
- Complexity      29326      29416        +90     
==================================================
  Files            1382       1388         +6     
  Lines          116604     116804       +200     
  Branches        20427      20446        +19     
==================================================
+ Hits            78135      78296       +161     
- Misses          32036      32069        +33     
- Partials         6433       6439         +6     

Files with missing lines	Coverage Δ
...g/apache/groovy/parser/antlr4/ModifierManager.java	93.4210% <100.0000%> (ø)
...ain/java/org/codehaus/groovy/ast/ModifierNode.java	77.7778% <100.0000%> (+0.4193%)	:arrow_up:
...va/org/codehaus/groovy/ast/tools/GeneralUtils.java	86.0412% <100.0000%> (+0.1943%)	:arrow_up:
...va/org/apache/groovy/parser/antlr4/AstBuilder.java	86.6667% <88.8889%> (+0.0444%)	:arrow_up:
...ehaus/groovy/transform/AsyncASTTransformation.java	80.9524% <80.9524%> (ø)
...ain/java/groovy/util/concurrent/async/Promise.java	0.0000% <0.0000%> (ø)
...a/groovy/util/concurrent/async/StageAwaitable.java	0.0000% <0.0000%> (ø)
...java/groovy/util/concurrent/async/AsyncHelper.java	70.0000% <70.0000%> (ø)
...a/groovy/util/concurrent/async/AwaitException.java	25.0000% <25.0000%> (ø)
...va/groovy/util/concurrent/async/SimplePromise.java	83.3333% <83.3333%> (ø)

... and 3 files with indirect coverage changes

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[daniellansun]

@codecov-commenter